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Electromagnetic coil

Electromagnets and Inductive Coils

Electromagnet . The physical thing which we call an electromagnet, consisting of a coil or helix of wire, the turns of which are insulated from each other, and within which is usually included an iron core, is by far the most useful of all the so-called translating devices employed in telephony. In performing the ordinary functions of an electromagnet it translates the energy of an electrical current into the energy of mechanical motion. An almost equally important function is the converse of this, that is, the translation of the energy of mechanical motion into that of an electrical current. In addition to these primary functions which underlie the art of telephony, the electromagnetic coil or helix serves a wide field of usefulness in cases where no mechanical motion is involved. As impedance coils, they serve to exert important influences on the flow of currents in circuits, and as induction coils, they serve to translate the energy of a current flowing in one circuit into the energy of a current flowing in another circuit, the translation usually, but not always, being accompanied by a change in voltage.

When a current flows through the convolutions of an ordinary helix, the helix will exhibit the properties of a magnet even though the substance forming the core of the helix is of non-magnetic material, such as air, or wood, or brass. If, however, a mass of iron, such as a rod or a bundle of soft iron wires, for instance, is substituted as a core, the magnetic properties will be enormously increased. The reason for this is, that a given magnetizing force will set up in iron a vastly greater number of lines of magnetic force than in air or in any other non-magnetic material.

Magnetizing Force. The magnetizing force of a given helix is that force which tends to drive magnetic lines of force through the magnetic circuit interlinked with the helix. It is called magnetomotive force  and is analogous to electromotive force, that is, the force which tends to drive an electric current through a circuit.

The magnetizing force of a given helix depends on the product of the current strength and the number of turns of wire in the helix. Thus, when the current strength is measured in amperes, this magnetizing force is expressed as ampere-turns, being the product of the number of amperes flowing by the number of turns. The magnetizing force exerted by a given current, therefore, is independent of anything except the number of turns, and the material within the core or the shape of the core has no effect upon it.

Magnetic Flux. The total magnetization resulting from a magnetizing force is called the magnetic flux, and is analogous to current. The intensity of a magnetic flux is expressed by the number of magnetic lines of force in a square centimeter or square inch.

While the magnetomotive force or magnetizing force of a given helix is independent of the material of the core, the flux which it sets up is largely dependent on the material and shape of the core—not only upon this but on the material that lies in the return path for the flux outside of the core. We may say, therefore, that the amount of flux set up by a given current in a given coil or helix is dependent on the material in the magnetic path or magnetic circuit, and on the shape and length of that circuit. If the magnetic circuit be of air or brass or wood or any other non-magnetic material, the amount of flux set up by a given magnetizing force will be relatively small, while it will be very much greater if the magnetic circuit be composed in part or wholly of iron or steel, which are highly magnetic substances.

Permeability . The quality of material, which permits of a given magnetizing force setting up a greater or less number of lines of force within it, is called its permeability. More accurately, the permeability is the ratio existing between the amount of magnetization and the magnetizing force which produces such magnetization.

The permeability of a substance is usually represented by the Greek letter µ (pronounced mu ). The intensity of the magnetizing force is commonly symbolized by H, and since the permeability of air is always taken as unity, we may express the intensity of magnetizing force by the number of lines of force per square centimeter which it sets up in air.

Now, if the space on which the given magnetizing force H were acting were filled with iron instead of air, then, owing to the greater permeability of iron, there would be set up a very much greater number of lines of force per square centimeter, and this number of lines of force per square centimeter in the iron is the measure of the magnetization produced and is commonly expressed by the letter B .

From this we have

µ = B ÷ H

Thus, when we say that the permeability of a given specimen of wrought iron under given conditions is 2,000, we mean that 2,000 times as many lines of force would be induced in a unit cross-section of this sample as would be induced by the same magnetizing force in a corresponding unit cross-section of air. Evidently for air B  = H , hence µ becomes unity.

The permeability of air is always a constant. This means that whether the magnetic density of the lines of force through the air be great or small the number of lines will always be proportional to the magnetizing force. Unfortunately for easy calculations in electromagnetic work, however, this is not true of the permeability of iron. For small magnetic densities the permeability is very great, but for large densities, that is, under conditions where the number of lines of force existing in the iron is great, the permeability becomes smaller, and an increase in the magnetizing force does not produce a corresponding increase in the total flux through the iron.

Magnetization Curves . This quality of iron is best shown by the curves of Fig. 89, which illustrate the degree of magnetization set up in various kinds of iron by different magnetizing forces. In these curves the ordinates represent the total magnetization B , while the abscissas represent the magnetizing force H . It is seen from an inspection of these curves that as the magnetizing force H  increases, the intensity of flux also increases, but at a gradually lessening rate, indicating a reduction in permeability at the higher densities. These curves are also instructive as showing the great differences that exist between the permeability of the different kinds of iron; and also as showing how, when the magnetizing force becomes very great, the iron approaches what is called saturation, that is, a point at which the further increase in magnetizing force will result in no further magnetization of the core.

From the data of the curves of Fig. 89, which are commonly called magnetization curves, it is easy to determine other data from which so-called permeability curves may be plotted. In permeability curves the total magnetization of the given pieces of iron are plotted as abscissas, while the corresponding permeabilities are plotted as ordinates.

Illustration: Fig. 89. Magnetization Curve 
Fig. 89. Magnetization Curve
View full size illustration.

Direction of Lines of Force. The lines of force set up within the core of a helix always have a certain direction. This direction always depends upon the direction of the flow of current around the core. An easy way to remember the direction is to consider the helix as grasped in the right hand with the fingers partially encircling it and the thumb pointing along its axis. Then, if the current through the convolutions of the helix be in the direction in which the fingers of the hand are pointed around the helix, the magnetic lines of force will proceed through the core of the helix along the direction in which the thumb is pointed.

In the case of a simple bar electromagnet, such as is shown in Fig. 90, the lines of force emerging from one end of the bar must pass back through the air to the other end of the bar, as indicated by dotted lines and arrows. The path followed by the magnetic lines of force is called the magnetic circuit, and, therefore, the magnetic circuit of the magnet shown in Fig. 90 is composed partly of iron and partly of air. From what has been said concerning the relative permeability of air and of iron, it will be obvious that the presence of such a long air path in the magnetic circuit will greatly reduce the number of lines of force that a given magnetizing force can set up. The presence of an air gap in a magnetic circuit has much the same effect on the total flow of lines of force as the presence of a piece of bad conductor in a circuit composed otherwise of good conductor, in the case of the flow of electric current.

Reluctance. As the property which opposes the flow of electric current in an electrical circuit is called resistance, so the property which opposes the flow of magnetic lines of force in a magnetic circuit is called reluctance. In the case of the electric circuit, the resistance is the reciprocal of the conductivity; in the case of the magnetic circuit, the reluctance is the reciprocal of the permeability. As in the case of an electrical circuit, the amount of flow of current is equal to the electromotive force divided by the resistance; so in a magnetic circuit, the magnetic flux is equal to the magnetizing force or magnetomotive force divided by the reluctance.

Illustration: Fig. 90. Bar Electromagnet 
Fig. 90. Bar Electromagnet
View full size illustration.

Types of Low-Reluctance Circuits. As the pull of an electromagnet upon its armature depends on the total number of lines of force passing from the core to the armature—that is, on the total flux—and as the total flux depends for a given magnetizing force on the reluctance of the magnetic circuit, it is obvious that the design of the electromagnetic circuit is of great importance in influencing the action of the magnet. Obviously, anything that will reduce the amount of air or other non-magnetic material that is in the magnetic circuit will tend to reduce the reluctance, and, therefore, to increase the total magnetization resulting from a given magnetizing force.

Horseshoe Form. One of the easiest and most common ways of reducing reluctance in a circuit is to bend the ordinary bar electromagnet into horseshoe form. In order to make clear the direction of current flow, attention is called to Fig. 91. This is intended to represent a simple bar of iron with a winding of one direction throughout its length. The gap in the middle of the bar, which divides the winding into two parts, is intended merely to mark the fact that the winding need not cover the whole length of the bar and still will be able to magnetize the bar when the current passes through it. In Fig. 92 a similar bar is shown with similar winding upon it, but bent into U -form, exactly as if it had been grasped in the hand and bent without further change. The magnetic polarity of the two ends of the bar remain the same as before for the same direction of current, and it is obvious that the portion of the magnetic circuit which extends through air has been very greatly shortened by the bending. As a result, the magnetic reluctance of the circuit has been greatly decreased and the strength of the magnet correspondingly increased.

Illustration: Fig. 91. Bar Electromagnet 
Fig. 91. Bar Electromagnet
View full size illustration.
Illustration: Fig. 92. Horseshoe Electromagnet 
Fig. 92. Horseshoe Electromagnet
View full size illustration.
Illustration: Fig. 93. Horseshoe Electromagnet 
Fig. 93. Horseshoe Electromagnet
View full size illustration.

If the armature of the electromagnet shown in Fig. 92 is long enough to extend entirely across the air gap from the south to the north pole, then the air gap in the magnetic circuit is still further shortened, and is now represented only by the small gap between the ends of the armature and the ends of the core. Such a magnet, with an armature closely approaching the poles, is called a closed-circuit magnet, since the only gap in the iron of the magnetic circuit is that across which the magnet pulls in attracting its armature.

In Fig. 93 is shown the electrical and magnetic counterpart of Fig. 92. The fact that the magnetic circuit is not a single iron bar but is made up of two cores and one backpiece rigidly secured together, has no bearing upon the principle, but only shows that a modification of construction is possible. In the construction of Fig. 93 the armature 1  is shown as being pulled directly against the two cores 2  and 3, these two cores being joined by a yoke 4, which, like the armature and the core, is of magnetic material. The path of the lines of force is indicated by dotted lines. This is a very important form of electromagnet and is largely used in telephony.

Iron-Clad Form. Another way of forming a closed-circuit magnet that is widely used in telephony is to enclose the helix or winding in a shell of magnetic material which joins the core at one end. This construction results in what is known as the tubular  or iron-clad  electromagnet, which is shown in section and in end view in Fig. 94. In this the core 1  is a straight bar of iron and it lies centrally within a cylindrical shell 2, also of iron. The bar is usually held in place within the shell by a screw, as shown. The lines of force set up in the core by the current flowing through the coil, pass to the center of the bottom of the iron shell and thence return through the metal of the shell, through the air gap between the edges of the shell and the armature, and then concentrate at the center of the armature and pass back to the end of the core. This is a highly efficient form of closed-circuit magnet, since the magnetic circuit is of low reluctance.

Illustration: Fig. 94. Iron-Clad Electromagnet 
Fig. 94. Iron-Clad Electromagnet
View full size illustration.

Such forms of magnets are frequently used where it is necessary to mount a large number of them closely together and where it is desired that the current flowing in one magnet shall produce no inductive effect in the coils of the adjacent magnets. The reason why mutual induction between adjacent magnets is obviated in the case of the iron-clad or tubular magnet is that practically all stray field is eliminated, since the return path for the magnetic lines is so completely provided for by the presence of the iron shell.

Special Horseshoe Form. In Fig. 95 is shown a type of relay commonly employed in telephone circuits. The purpose of illustrating it in this chapter is not to discuss relays, but rather to show an adaptation of an electromagnet wherein low reluctance of the magnetic circuit is secured by providing a return leg for the magnetic lines developed in the core, thus forming in effect a horseshoe magnet with a winding on one of its limbs only. To the end of the core 1  there is secured an L -shaped piece of soft iron 2. This extends upwardly and then forwardly throughout the entire length of the magnet core. An L -shaped armature 3  rests on the front edge of the piece 2  so that a slight rocking motion will be permitted on the "knife-edge" bearing thus afforded. It is seen from the dotted lines that the magnetic circuit is almost a closed one. The only gap is that between the lower end of the armature 3  and the front end of the core. When the coil is energized, this gap is closed by the attraction of the armature. As a result, the rearwardly projecting end of the armature 3  is raised and this raises the spring 4  and causes it to break the normally existing contact with the spring 5  and to establish another contact with the spring 6. Thus the energy developed within the coil of the magnet is made to move certain parts which in turn operate the switching devices to produce changes in electrical circuits. These relays and other adaptations of the electromagnet will be discussed more fully later on.

Illustration: Fig. 95. Electromagnet of Relay 
Fig. 95. Electromagnet of Relay
View full size illustration.

There are almost numberless forms of electromagnets, but we have illustrated here examples of the principal types employed in telephony, and the modifications of these types will be readily understood in view of the general principles laid down.

Direction of Armature Motion . It may be said in general that the armature of an electromagnet always moves or tends to move, when the coil is energized, in such a way as to reduce the reluctance of the magnetic circuit through the coil. Thus, in all of the forms of electromagnets discussed, the armature, when attracted, moves in such a direction as to shorten the air gap and to introduce the iron of the armature as much as possible into the path of the magnetic lines, thus reducing the reluctance. In the case of a solenoid type of electromagnet, or the coil and plunger type, which is a better name than solenoid, the coil, when energized, acts in effect to suck the iron core or plunger within itself so as to include more and more of the iron within the most densely occupied portion of the magnetic circuit.

Illustration: Fig. 96. Parallel Differential Electromagnet 
Fig. 96. Parallel Differential Electromagnet
View full size illustration.

Differential Electromagnet . Frequently in telephony, the electromagnets are provided with more than one winding. One purpose of the double-wound electromagnet is to produce the so-called differential action between the two windings, i.e., making one of the windings develop magnetization in the opposite direction from that of the other, so that the two will neutralize each other, or at least exert different and opposite influences. The principle of the differential electromagnet may be illustrated in connection with Fig. 96. Here two wires 1  and 2  are shown wrapped in the same direction about an iron core, the ends of the wire being joined together at 3. Obviously, if one of these windings only is employed and a current sent through it, as by connecting the terminals of a battery with the points 4  and 3, for instance, the core will be magnetized as in an ordinary magnet. Likewise, the core will be energized if a current be sent from 5  to 3. Assuming that the two windings are of equal resistance and number of turns, the effects so produced, when either the coil 1  or the coil 2  is energized, will be equal. If the battery be connected between the terminals 4  and 5  with the positive pole, say, at 5, then the current will proceed through the winding 2  and tend to generate magnetism in the core in the direction of the arrow. After traversing the winding 2, however, it will then begin to traverse the other winding 1  and will pass around the core in the opposite direction throughout the length of that winding. This will tend to set up magnetism in the core in the opposite direction to that indicated by the arrow. Since the two currents are equal and also the number of turns in each winding, it is obvious that the two magnetizing influences will be exactly equal and opposite and no magnetic effect will be produced. Such a winding, as is shown in Fig. 96, where the two wires are laid on side by side, is called a parallel differential winding.

Another way of winding magnets differentially is to put one winding on one end of the core and the other winding on the other end of the core and connect these so as to cause the currents through them to flow around the core in opposite directions. Such a construction is shown in Fig. 97 and is called a tandem differential winding. The tandem arrangement, while often good enough for practical purposes, cannot result in the complete neutralization of magnetic effect. This is true because of the leakage of some of the lines of force from intermediate points in the length of the core through the air, resulting in some of the lines passing through more of the turns of one coil than of the other. Complete neutralization can only be attained by first twisting the two wires together with a uniform lay and then winding them simultaneously on the core.

Illustration: Fig. 97. Tandem Differential Electromagnet 
Fig. 97. Tandem Differential Electromagnet
View full size illustration.

Mechanical Details. We will now consider the actual mechanical construction of the electromagnet. This is a very important feature of telephone work, because, not only must the proper electrical and magnetic effects be produced, but also the whole structure of the magnet must be such that it will not easily get out of order and not be affected by moisture, heat, careless handling, or other adverse conditions.

The most usual form of magnet construction employed in telephony is shown in Fig. 98. On the core, which is of soft Norway iron, usually cylindrical in form, are forced two washers of either fiber or hard rubber. Fiber is ordinarily to be preferred because it is tougher and less liable to breakage. Around the core, between the two heads, are then wrapped several layers of paper or specially prepared cloth in order that the wire forming the winding may be thoroughly insulated from the core. One end of the wire is then passed through a hole in one of the spool heads or washers, near the core, and the wire is then wound on in layers. Sometimes a thickness of paper is placed around each layer of wire in order to further guard against the breaking down of the insulation between layers. When the last layer is wound on, the end of the wire is passed out through a hole in the head, thus leaving both ends projecting.

Illustration: Fig. 98 Construction of Electromagnet 
Fig. 98 Construction of Electromagnet
View full size illustration.

Magnet Wire. The wire used in winding magnets is, of course, an important part of the electromagnet. It is always necessary that the adjacent turns of the wire be insulated from each other so that the current shall be forced to pass around the core through all the length of wire in each turn rather than allowing it to take the shorter and easier path from one turn to the next, as would be the case if the turns were not insulated. For this purpose the wire is usually covered with a coating of some insulating material. There are, however, methods of winding magnet coils with bare wire and taking care of the insulation between the turns in another way, as will be pointed out.

Insulated wire for the purpose of winding magnet coils is termed magnet wire. Copper is the material almost universally employed for the conductor. Its high conductivity, great ductility, and low cost are the factors which make it superior to all other metals. However, in special cases, where exceedingly high conductivity is required with a limited winding space, silver wire is sometimes employed, and on the other hand, where very high resistance is desired within a limited winding space, either iron or German silver or some other high-resistance alloy is used.

Wire Gauges. Wire for electrical purposes is drawn to a number of different standard gauges. Each of the so-called wire gauges consists of a series of graded sizes of wire, ranging from approximately one-half an inch in diameter down to about the fineness of a lady's hair. In certain branches of telephone work, such as line construction, the existence of the several wire gauges or standards is very likely to lead to confusion. Fortunately, however, so far as magnet wire is concerned, the so-called Brown and Sharpe, or American, wire gauge is almost universally employed in this country. The abbreviations for this gauge are B.&S. or A.W.G.


Copper Wire Table

Giving weights, lengths, and resistances of wire @ 68° F., of Matthiessen's Standard Conductivity.

Ohms per Pound Ohms Per Foot Feet per Pound Feet per Ohm Pounds per Foot Pounds per Ohm
0000 460.211,600.0.00007639 0.0000489 1.561 20,440.0.6405 13,090.
000 409.6 167,800.0.0001215 0.0000617 1.969 16,210.0.5080 8,232.
00 364.8 133,100.0.0001931 0.0000778 2.482 12,850.0.4028 5,177.
0 324.9 105,500.0.0003071 0.0000981 3.130 10,190.0.3195 3,256.
1 289.3 83,690.0.0004883 0.0001237 3.947 8,083.0.2533 2,048.
2 257.6 66,370.0.0007765 0.0001560 4.977 6,410.0.2009 1,288.
3 229.4 52,630.0.001235 0.0001967 6.276 5,084.0.1593 810.0
4 204.3 41,740.0.001963 0.0002480 7.914 4,031.0.1264 509.4
5 181.9 33,100.0.003122 0.0003128 9.980 3,197.0.1002 320.4
6 162.0 26,250.0.004963 0.0003944 12.58 2,535.0.07946 201.5
7 144.3 20,820.0.007892 0.0004973 15.87 2,011.0.06302 126.7
8 128.5 16,510.0.01255 0.0006271 20.01 1,595.0.04998 79.69
9 114.4 13,090.0.01995 0.0007908 25.23 1,265.0.03963 50.12
10 101.9 10,380.0.03173 0.0009273 31.82 1,003.0.03143 31.52
11 90.74 8,234.0.05045 0.001257 40.12 795.3 0.02493 19.82
12 80.81 6,530.0.08022 0.001586 50.59 630.7 0.01977 12.47
13 71.96 5,178.0.1276 0.001999 63.79 500.1 0.01568 7.840
14 64.08 4,107.0.2028 0.002521 80.44 396.6 0.01243 4.931
15 57.07 3,257.0.3225 0.003179 101.4 314.5 0.009858 3.101
16 50.82 2,583.0.5128 0.004009 127.9 249.4 0.007818 1.950
17 45.26 2,048.0.8153 0.005055 161.3 197.8 0.006200 1.226
18 40.30 1,624.1.296 0.006374 203.4 156.9 0.004917 0.7713
19 35.89 1,288.2.061 0.008038 256.5 124.4 0.003899 0.4851
20 31.96 1,022.3.278 0.01014 323.4 98.66 0.003092 0.3051
21 28.46 810.1 5.212 0.01278 407.8 78.24 0.002452 0.1919
22 25.35 642.4 8.287 0.01612 514.2 62.05 0.001945 0.1207
23 22.57 509.5 13.18 0.02032 648.4 49.21 0.001542 0.07589
24 20.10 404.0 20.95 0.02563 817.6 39.02 0.001223 0.04773
25 17.90 320.4 33.32 0.03231 1,031.30.95 0.0009699 0.03002
26 15.94 254.1 52.97 0.04075 1,300.24.54 0.0007692 0.1187
27 14.2 201.5 84.23 0.05138 1,639.19.46 0.0006100 0.01888
28 12.64 159.8 133.9 0.06479 2,067.15.43 0.0004837 0.007466
29 11.26 126.7 213.0 0.08170 2,607.12.24 0.0003836 0.004696
30 10.03 100.5 338.6 0.1030 3,287.9.707 0.0003042 0.002953
31 8.928 79.70 538.4 0.1299 4,145.7.698 0.0002413 0.001857
32 7.950 63.21 856.2 0.1638 5,227.6.105 0.0001913 0.001168
33 7.080 50.13 1,361.0.2066 6,591.4.841 0.0001517 0.0007346
34 6.305 39.75 2,165.0.2605 8,311.3.839 0.0001203 0.0004620
35 5.615 31.52 3,441.0.3284 10,480.3.045 0.00009543 0.0002905
36 5.0 25.0 5,473.0.4142 13,210.2.414 0.00007568 0.0001827
37 4.453 19.83 8,702.0.5222 16,660.1.915 0.00006001 0.0001149
38 3.965 15.72 13,870.0.6585 21,010.1.519 0.00004759 0.00007210
39 3.531 12.47 22,000.0.8304 26,500.1.204 0.00003774 0.00004545
40 3.145 9.888 34,980.1.047 33,410.0.9550 0.00002993 0.00002858


In the Brown and Sharpe gauge the sizes, beginning with the largest, are numbered 0000, 000, 00, 0, 1, 2, and so on up to 40. Sizes larger than about No. 16 B.&S. gauge are seldom used as magnet wire in telephony, but for the purpose of making the list complete, Table III is given, including all of the sizes of the B.&S. gauge.

In Table III there is given for each gauge number the diameter of the wire in mils (thousandths of an inch); the cross-sectional area in circular mils (a unit area equal to that of a circle having a diameter of one one-thousandth of an inch); the resistance of the wire in various units of length and weight; the length of the wire in terms of resistance and of weight; and the weight of the wire in terms of its length and resistance.

It is to be understood that in Table III the wire referred to is bare wire and is of pure copper. It is not commercially practicable to use absolutely pure copper, and the ordinary magnet wire has a conductivity equal to about 98 per cent of that of pure copper. The figures given in this table are sufficiently accurate for all ordinary practical purposes.

Silk and Cotton Insulation. The insulating material usually employed for covering magnet wire is of silk or cotton. Of these, silk is by far the better material for all ordinary purposes, since it has a much higher insulating property than cotton, and is very much thinner. Cotton, however, is largely employed, particularly in the larger sizes of magnet wire. Both of these materials possess the disadvantage of being hygroscopic, that is, of readily absorbing moisture. This disadvantage is overcome in many cases by saturating the coil after it is wound in some melted insulating compound, such as wax or varnish or asphaltum, which will solidify on cooling. Where the coils are to be so saturated the best practice is to place them in a vacuum chamber and exhaust the air, after which the hot insulating compound is admitted and is thus drawn into the innermost recesses of the winding space.

Silk-insulated wire, as regularly produced, has either one or two layers of silk. This is referred to commercially as single silk wire or as double silk wire. The single silk has a single layer of silk fibers wrapped about it, while the double silk has a double layer, the two layers being put on in reverse direction. The same holds true of cotton insulated wire. Frequently, also, there is a combination of the two, consisting of a single or a double wrapping of silk next to the wire with an outer wrapping of cotton. Where this is done the cotton serves principally as a mechanical protection for the silk, the principal insulating properties residing in the silk.

Enamel. A later development in the insulation of magnet wire has resulted in the so-called enamel wire. In this, instead of coating the wire with some fibrous material such as silk or cotton, the wire is heated and run through a bath of fluid insulating material or liquid enamel, which adheres to the wire in a very thin coating. The wire is then run through baking ovens, so that the enamel is baked on. This process is repeated several times so that a number of these thin layers of the enamel are laid on and baked in succession.

The characteristics sought in good enamel insulation for magnet wire may be thus briefly set forth: It is desirable for the insulation to possess the highest insulating qualities; to have a glossy, flawless surface; to be hard without being brittle; to adhere tenaciously and stand all reasonable handling without cracking or flaking; to have a coefficient of elasticity greater than the wire itself; to withstand high temperatures; to be moisture-proof and inert to corrosive agencies; and not to "dry out" or become brittle over a long period of time.

Space Utilization. The utilization of the winding space in an electromagnet is an important factor in design, since obviously the copper or other conductor is the only part of the winding that is effective in setting up magnetizing force. The space occupied by the insulation is, in this sense, waste space. An ideally perfect winding may be conceived as one in which the space is all occupied by wire; and this would necessarily involve the conception of wire of square cross-section and insulation of infinite thinness. In such a winding there would be no waste of space and a maximum amount of metal employed as a conductor. Of course, such a condition is not possible to attain and in practice some insulating material must be introduced between the layers of wire and between the adjacent convolutions of wire. The ratio of the space occupied by the conductor to the total space occupied by the winding, that is, by the conductor and the insulation, is called the coefficient of space utilization of the coil. For the ideal coil just conceived the coefficient of space utilization would be 1. Ordinarily the coefficient of space utilization is greater for coarse wire than for fine wire, since obviously the ratio of the diameter of the wire to the thickness of the insulation increases as the size of the wire grows larger.

The chief advantage of enamel insulation for magnet wire is its thinness, and the high coefficient of space utilization which may be secured by its use. In good enamel wire the insulation will average about one-quarter the thickness of the standard single silk insulation, and the dielectric strength is equal or greater. Where economy of winding space is desirable the advantages of this may readily be seen. For instance, in a given coil wound with No. 36 single silk wire about one-half of the winding space is taken up with the insulation, whereas when the same coil is wound with No. 36 enameled wire only about one-fifth of the winding space is taken up by the insulation. Thus the coefficient of space utilization is increased from .50 to .80. The practical result of this is that, in the case of any given winding space where No. 36 wire is used, about 60 per cent more turns can be put on with enameled wire than with single silk insulation, and of course this ratio greatly increases when the comparison is made with double silk insulation or with cotton insulation. Again, where it is desired to reduce the winding space and keep the same number of turns, an equal number of turns may be had with a corresponding reduction of winding space where enameled wire is used in place of silk or cotton.

In the matter of heat-resisting properties the enameled wire possesses a great advantage over silk and cotton. Cotton or silk insulation will char at about 260° Fahrenheit, while good enameled wire will stand 400° to 500° Fahrenheit without deterioration of the insulation. It is in the matter of liability to injury in rough or careless handling, or in winding coils having irregular shapes, that enamel wire is decidedly inferior to silk or cotton-covered wire. It is likely to be damaged if it is allowed to strike against the sharp corners of the magnet spool during winding, or run over the edge of a hard surface while it is being fed on to the spool. Coils having other than round cores, or having sharp corners on their spool heads, should not ordinarily be wound with enamel wire.

The dielectric strength of enamel insulation is much greater than that of either silk or cotton insulation of equal thickness. This is a distinct advantage and frequently a combination of the two kinds of insulation results in a superior wire. If wire insulated with enamel is given a single wrapping of silk or of cotton, the insulating and dielectric properties of the enamel is secured, while the presence of the silk and cotton affords not only an additional safeguard against bare spots in the enamel but also a certain degree of mechanical protection to the enamel.

Winding Methods . In winding a coil, the spool, after being properly prepared, is placed upon a spindle which may be made to revolve rapidly. Ordinarily the wire is guided on by hand; sometimes, however, machinery is used, the wire being run over a tool which moves to and fro along the length of the spool, just fast enough to lay the wire on at the proper rate. The movement of this tool is much the same as that of the tool in a screw cutting lathe.

Unless high voltages are to be encountered, it is ordinarily not necessary to separate the layers of wire with paper, in the case of silk-or cotton-insulated magnet wire; although where especially high insulation resistance is needed this is often done. It is necessary to separate the successive layers of a magnet that is wound with enamel wire, by sheets of paper or thin oiled cloth.

Illustration: Fig. 99. Electromagnet with Bare Wire 
Fig. 99. Electromagnet with Bare Wire
View full size illustration.

In Fig. 99 is shown a method, that has been used with some success, of winding magnets with bare wire. In this the various adjacent turns are separated from each other by a fine thread of silk or cotton wound on beside the wire. Each layer of wire and thread as it is placed on the core is completely insulated from the subsequent layer by a layer of paper. This is essentially a machine-wound coil, and machines for winding it have been so perfected that several coils are wound simultaneously, the paper being fed in automatically at the end of each layer.

Another method of winding the bare wire omits the silk thread and depends on the permanent positioning of the wire as it is placed on the coil, due to the slight sinking into the layer of paper on which it is wound. In this case the feed of the wire at each turn of the spool is slightly greater than the diameter of the wire, so that a small distance will be left between each pair of adjacent turns.

Upon the completion of the winding of a coil, regardless of what method is used, it is customary to place a layer of bookbinders' cloth over the coil so as to afford a certain mechanical protection for the insulated wire.

Winding Terminals. The matter of bringing out the terminal ends of the winding is one that has received a great deal of attention in the construction of electromagnets and coils for various purposes. Where the winding is of fine wire, it is always well to reinforce its ends by a short piece of larger wire. Where this is done the larger wire is given several turns around the body of the coil, so that the finer wire with which it connects may be relieved of all strain which may be exerted upon it from the protruding ends of the wire. Great care is necessary in the bringing out of the inner terminal—i.e., the terminal which connects with the inner layer—that the terminal wire shall not come in contact with any of the subsequent layers that are wound on.

Illustration: Fig. 100. Electromagnet with Terminals 
Fig. 100. Electromagnet with Terminals
View full size illustration.

Where economy of space is necessary, a convenient method of terminating the winding of the coil consists in fastening rigid terminals to the spool head. This, in the case of a fiber spool head, may be done by driving heavy metal terminals into the fiber. The connections of the two wires leading from the winding are then made with these heavy rigid terminals by means of solder. A coil having such terminals is shown in its finished condition in Fig. 100.

Winding Data. The two things principally affecting the manufacture of electromagnets for telephone purposes are the number of turns in a winding  and the resistance of the wound wire. The latter governs the amount of current which may flow through the coil with a given difference of potential at its end, while the former control the amount of magnetism produced in the core by the current flowing. While a coil is being wound, it is a simple matter to count the turns by any simple form of revolution counter. When the coil has been completed it is a simple matter to measure its resistance. But it is not so simple to determine in advance how many turns of a given size wire may be placed on a given spool, and still less simple to know what the resistance of the wire on that spool will be when the desired turns shall have been wound.


Winding Data for Insulated Wires—Silk and Cotton Covering

A.W.G. B & S 20 21 22 23 24 25 26
DIAMETER Mils 31.961 28.462 25.347 22.571 20.100 17.900 15.940
AREA Circular Mils 1021.20 810.10 642.70 509.45 404.01 320.40 254.01
Single Cotton 37.861 34.362 31.247 28.471 26.000 23.800 21.840
Double Cotton 42.161 38.662 35.547 32.771 30.300 28.100 26.140
Single Silk 34.261 30.762 27.647 24.871 22.401 20.200 18.240
Double Silk 36.161 32.662 29.547 26.771 24.300 22.100 20.140
Single Cotton 25.7 28.3 31.0 34.4 36.9 38.0 42.0
Double Cotton 22.5 24.5 26.7 28.97 31.35 33.92 36.29
Single Silk 27.70 30.97 34.39 38.19 42.37 47.02 52.06
Double Silk 26.22 29.07 32.11 35.53 39.14 42.94 46.81
Single Cotton 660.5 800.9 961.0 1183.0 1321.6 1444.0 1764.0
Double Cotton 506.3 600.2 712.9 839.2 982.8 1150.8 1317.0
Single Silk 767.3 959.1 1182.7 1458.5 1795.2 2210.9 2710.3
Double Silk 687.5 845.0 1031.0 1262.4 1532.0 1843.8 2191.2
Single Cotton .646 .981 1.502 2.359 3.528 5.831 6.941
Double Cotton .533 .795 1.188 1.772 2.595 3.802 5.552
Single Silk .801 1.261 1.956 3.049 4.739 7.489 9.031


A.W.G. B & S 27 28 29 30 31 32 33
DIAMETER Mils 14.195 12.641 11.257 10.025 8.928 7.950 7.080
AREA Circular Mils 201.50 159.79 126.72 100.50 79.71 63.20 50.13
Single Cotton 20.095 18.541 17.157 15.925 14.828 13.850 12.980
Double Cotton 24.395 22.841 21.457 20.225 19.128 18.150 17.280
Single Silk 16.495 14.941 13.557 12.325 11.228 10.250 9.380
Double Silk 18.395 16.841 15.457 14.225 13.128 12.150 11.280
Single Cotton 48.0 53.0 56.5 59.66 64.125 68.600 73.050
Double Cotton 38.95 41.61 44.27 46.93 49.78 52.34 55.10
Single Silk 57.67 63.36 70.11 77.14 84.64 92.72 101.65
Double Silk 51.59 56.43 61.56 66.79 72.39 78.19 84.17
Single Cotton 2304.0 2809.9 3192.3 3359.2 4112.2 4692.5 5333.5
Double Cotton 1517.2 1731.0 1959.9 2202.5 2478.0 2739.5 3036.1
Single Silk 3326.0 4014.5 4915.5 5950.2 7164.0 8597.5 10332.0
Double Silk 2661.6 3184.5 3789.8 4461.0 5240.0 6114.0 7085.0
Single Cotton 10.814 17.617 25.500 34.800 48.5 73.8 104.5
Double Cotton 8.078 11.54 16.47 23.43 32.83 46.19 64.30
Single Silk 13.92 26.86 41.29 62.98 95.70 144.70 217.8


A.W.G. B & S 34 35 36 37 38 39 40
DIAMETER Mils 6.304 5.614 5.000 4.453 3.965 3.531 3.144
AREA Circular Mils 39.74 31.52 25.00 19.83 15.72 12.47 9.89
Single Cotton 12.204 11.514 1090.0 10.353 9.865 9.431 9.044
Double Cotton 16.504 15.814 15.200 14.653 14.165 13.731 13.344
Single Silk 8.504 7.914 7.300 6.753 6.265 5.831 5.344
Double Silk 10.504 9.814 9.200 8.653 8.165 7.731 7.344
Single Cotton 77.900 82.600 87.100 91.870 95.000 100.700 106.000
Double Cotton 57.57 60.04 62.51 64.70 66.80 68.80 71.20
Single Silk 112.11 119.7 130.15 140.6 151.05 163.04 177.65
Double Silk 90.44 96.90 103.55 110.20 116.85 122.55 129.20
Single Cotton 6068.5 6773.3 7586.5 8440.0 9025.0 10140.5 11236.0
Double Cotton 3314.2 3605.0 3907.5 4186.1 4462.2 4733.6 5069.8
Single Silk 8179.5 9389.5 16940.0 19770.0 22820.0 26700.0 31559.0
Double Silk 8179.5 9389.5 10772.0 12145.0 13665.0 15018.0 16692.0
Single Cotton 151.4 202.0 298.8 418.0 567.0 811.0 1113.0
Double Cotton 70.58 125.9 166.3 225.6 305.5 409.8 545.5
Single Silk 342.1 489.0 721.1 1062.0 1557.0 2266.0 3400.0


If the length and the depth of the winding space of the coil as well as the diameter of the core are known, it is not difficult to determine how much bare copper wire of a given size may be wound on it, but it is more difficult to know these facts concerning copper wire which has been covered with cotton or silk. Yet something may be done, and tables have been prepared for standard wire sizes with definite thicknesses of silk and cotton insulation. As a result of facts collected from a large number of actually wound coils, the number of turns per linear inch and per square inch of B.&S. gauge wires from No. 20 to No. 40 have been tabulated, and these, supplemented by a tabulation of the number of ohms per cubic inch of winding space for wires of three different kinds of insulation, are given in Table IV.

Bearing in mind that the calculations of Table IV are all based upon the "diameter over insulation," which it states at the outset for each of four different kinds of covering, it is evident what is meant by "turns per linear inch." The columns referring to "turns per square inch" mean the number of turns, the ends of which would be exposed in one square inch if the wound coil were cut in a plane passing through the axis of the core. Knowing the distance between the head, and the depth to which the coil is to be wound, it is easy to select a size of wire which will give the required number of turns in the provided space. It is to be noted that the depth of winding space is one-half of the difference between the core diameter and the complete diameter of the wound coil. The resistance of the entire volume of wound wire may be determined in advance by knowing the total cubic contents of the winding space and multiplying this by the ohms per cubic inch of the selected wire; that is, one must multiply in inches the distance between the heads of the spool by the difference between the squares of the diameters of the core and the winding space, and this in turn by .7854. This result, times the ohms per cubic inch, as given in the table, gives the resistance of the winding.

There is a considerable variation in the method of applying silk insulation to the finer wires, and it is in the finer sizes that the errors, if any, pile up most rapidly. Yet the table throughout is based on data taken from many samples of actual coil winding by the present process of winding small coils. It should be said further that the table does not take into account the placing of any layers of paper between the successive layers of the wires. This table has been compared with many examples and has been used in calculating windings in advance, and is found to be as close an approximation as is afforded by any of the formulas on the subject, and with the further advantage that it is not so cumbersome to apply.

Winding Calculations. In experimental work, involving the winding of coils, it is frequently necessary to try one winding to determine its effect in a given circuit arrangement, and from the knowledge so gained to substitute another just fitted to the conditions. It is in such a substitution that the table is of most value. Assume a case in which are required a spool and core of a given size with a winding of, say No. 25 single silk-covered wire, of a resistance of 50 ohms. Assume also that the circuit regulations required that this spool should be rewound so as to have a resistance of, say 1,000 ohms. What size single silk-covered wire shall be used? Manifestly, the winding space remains the same, or nearly so. The resistance is to be increased from 50 to 1,000 ohms, or twenty times its first value. Therefore, the wire to be used must show in the table twenty times as many ohms per cubic inch as are shown in No. 25, the known first size. This amount would be twenty times 7.489, which is 149.8, but there is no size giving this exact resistance. No. 32, however, is very nearly of that resistance and if wound to exactly the same depth would give about 970 ohms. A few turns more would provide the additional thirty ohms.

Similarly, in a coil known to possess a certain number of turns, the table will give the size to be selected for rewinding to a greater or smaller number of turns. In this case, as in the case of substituting a winding of different resistance, it is unnecessary to measure and calculate upon the dimensions of the spool and core. Assume a spool wound with No. 30 double silk-covered wire, which requires to be wound with a size to double the number of turns. The exact size to do this would have 8922. turns per square inch and would be between No. 34 and No. 35. A choice of these two wires may be made, using an increased winding depth with the smaller wire and a shallower winding depth for the larger wire.

Impedance Coils. In telephony electromagnets frequently serve, as already stated, to perform other functions than the producing of motion by attracting or releasing their armatures. They are required to act as impedance coils to present a barrier to the passage of alternating or other rapidly fluctuating currents, and at the same time to allow the comparatively free passage of steady currents. Where it is desired that an electromagnet coil shall possess high impedance, it is usual to employ a laminated instead of a solid core. This is done by building up a core of suitable size by laying together thin sheets of soft iron, or by forming a bundle of soft iron wires. The use of laminated cores is for the purpose of preventing eddy currents, which, if allowed to flow, would not only be wasteful of energy but would also tend to defeat the desired high impedance. Sometimes in iron-clad impedance coils, the iron shell is slotted longitudinally to break up the flow of eddy currents in the shell.

Frequently electromagnetic coils have only the function of offering impedance, where no requirements exist for converting any part of the electric energy into mechanical work. Where this is the case, such coils are termed impedance, or retardation, or choke coils, since they are employed to impede or to retard or to choke back the flow of rapidly varying current. The distinction, therefore, between an impedance coil and the coil of an ordinary electromagnet is one of function, since structurally they may be the same, and the same principles of design and construction apply largely to each.

Number of Turns. It should be remembered that an impedance coil obstructs the passage of fluctuating current, not so much by ohmic resistance as by offering an opposing or counter-electromotive force. Other things being equal, the counter-electromotive force of self-induction increases directly as the number of turns on a coil and directly as the number of lines of force threading the coil, and this latter factor depends also on the reluctance of the magnetic circuit. Therefore, to secure high impedance we need many turns or low reluctance, or both. Often, owing to requirements for direct-current carrying capacity and limitations of space, a very large number of turns is not permissible, in which case sufficiently high impedance to such rapid fluctuations as those of voice currents may be had by employing a magnetic circuit of very low reluctance, usually a completely closed circuit.

Kind of Iron. An important factor in the design of impedance coils is the grade of iron used in the magnetic circuit. Obviously, it should be of the highest permeability and, furthermore, there should be ample cross-section of core to prevent even an approach to saturation. The iron should, if possible, be worked at that density of magnetization at which it has the highest permeability in order to obtain the maximum impedance effects.

Types. Open-Circuit:—Where very feeble currents are being dealt with, and particularly where there is no flow of direct current, an open magnetic circuit is much used. An impedance coil having an open magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its external appearance and illustrating particularly the method of bringing out the terminals of the winding.

Illustration: Fig. 101. Section of Open-Circuit Impedance Coil 
Fig. 101. Section of Open-Circuit Impedance Coil
View full size illustration.
Illustration: Fig. 102. Open-Circuit Impedance Coil 
Fig. 102. Open-Circuit Impedance Coil
View full size illustration.
Illustration: Fig. 103. Closed-Circuit Impedance Coil 
Fig. 103. Closed-Circuit Impedance Coil
View full size illustration.

Closed-Circuit:—A type of retardation coil which is largely used in systems of simultaneous telegraphy and telephony, known as composite systems, is shown in Fig. 103. In the construction of this coil the core is made of a bundle of fine iron wires first bent into U-shape, and then after the coils are in place, the free ends of the core are brought together to form a closed magnetic circuit. The coils have a large number of turns of rather coarse wire. The conditions surrounding the use of this coil are those which require very high impedance and rather large current-carrying capacity, and fortunately the added requirement, that it shall be placed in a very small space, does not exist.

Toroidal:—Another type of retardation coil, called the toroidal type due to the fact that its core is a torus formed by winding a continuous length of fine iron wire, is shown in diagram in Fig. 104. The two windings of this coil may be connected in series to form in effect a single winding, or it may be used as a "split-winding" coil, the two windings being in series but having some other element, such as a battery, connected between them in the circuit. Evidently such a coil, however connected, is well adapted for high impedance, on account of the low reluctance of its core.

Illustration: Fig. 104. Symbol of Toroidal Impedance Coil 
Fig. 104. Symbol of Toroidal Impedance Coil
View full size illustration.

This coil is usually mounted on a base-board, the coil being enclosed in a protecting iron case, as shown in Fig. 105. The terminal wires of both windings of each coil are brought out to terminal punchings on one end of the base-board to facilitate the making of the necessary circuit connections.

Illustration: Fig. 105. Toroidal Impedance Coil 
Fig. 105. Toroidal Impedance Coil
View full size illustration.

The usual diagrammatic symbol for an impedance coil is shown in Fig. 106. This is the same as for an ordinary bar magnet, except that the parallel lines through the core may be taken as indicating that the core is laminated, thus conveying the idea of high impedance. The symbol of Fig. 104 is a good one for the toroidal type of impedance coil.

Illustration: Fig. 106. Symbol of Impedance Coil 
Fig. 106. Symbol of Impedance Coil
View full size illustration.

Induction Coil. An induction coil consists of two or more windings of wire interlinked by a common magnetic circuit. In an induction coil having two windings, any change in the strength of the current flowing in one of the windings, called the primary, will cause corresponding changes in the magnetic flux threading the magnetic circuit, and, therefore, changes in flux through the other winding, called the secondary. This, by the laws of electromagnetic induction, will produce corresponding electromotive forces in the secondary winding and, therefore, corresponding currents in that winding if its circuit be closed.

Current and Voltage Ratios. In a well-designed induction coil the energy in the secondary, i.e., the induced current, is for all practical purposes equal to that of the primary current, yet the values of the voltage and the amperage of the induced current may vary widely from the values of the voltage and the amperage of the primary current. With simple periodic currents, such as the commercial alternating lighting currents, the ratio between the voltage in the primary and that in the secondary will be equal to the ratio of the number of turns in the primary to the number of turns in the secondary. Since the energy in the two circuits will be practically the same, it follows that the ratio between the current in the primary and that in the secondary will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary. In telephony, where the currents are not simple periodic currents, and where the variations in current strength take place at different rates, such a law as that just stated does not hold for all cases; but it may be stated in general that the induced currents will be of higher voltage and smaller current strength than those of the primary in all coils where the secondary winding has a greater number of turns than the primary, and vice versâ.

Functions. The function of the induction coil in telephony is, therefore, mainly one of transformation, that is, either of stepping up the voltage of a current, or in other cases stepping it down. The induction coil, however, does serve another purpose in cases where no change in voltage and current strength is desired, that is, it serves as a means for electrically separating two circuits so far as any conductive relation exists, and yet of allowing the free transmission by induction from one of these circuits to the other. This is a function that in telephony is scarcely of less importance than the purely transforming function.

Design. Induction coils, as employed in telephony, may be divided into two general types: first, those having an open magnetic circuit; and, second, those having a closed magnetic circuit. In the design of either type it is important that the core should be thoroughly laminated, and this is done usually by forming it of a bundle of soft Swedish or Norway iron wire about .02 of an inch in diameter. The diameter and the length of the coil, and the relation between the number of turns in the primary and in the secondary, and the mechanical construction of the coil, are all matters which are subject to very wide variation in practice. While the proper relationship of these factors is of great importance, yet they may not be readily determined except by actual experiment with various coils, owing to the extreme complexity of the action which takes place in them and to the difficulty of obtaining fundamental data as to the existing facts. It may be stated, therefore, that the design of induction coils is nearly always carried out by "cut-and-try" methods, bringing to bear, of course, such scientific and practical knowledge as the experimenter may possess.

Illustration: Fig. 107. Induction Coil 
Fig. 107. Induction Coil
View full size illustration.
Illustration: Fig. 108. Section of Induction Coil 
Fig. 108. Section of Induction Coil
View full size illustration.

Use and Advantage. The use and advantages of the induction coil in so-called local-battery telephone sets have already been explained in previous chapters. Such induction coils are nearly always of the open magnetic circuit type, consisting of a long, straight core comprised of a bundle of small annealed iron wires, on which is wound a primary of comparatively coarse wire and having a small number of turns, and over which is wound a secondary of comparatively fine wire and having a very much larger number of turns. A view of such a coil mounted on a base is shown in Fig. 107, and a sectional view of a similar coil is shown in Fig. 108. The method of bringing out the winding terminals is clearly indicated in this figure, the terminal wires 2  and 4  being those of the primary winding and 1  and 3  those of the secondary winding. It is customary to bring out these wires and attach them by solder to suitable terminal clips. In the case of the coil shown in Fig. 108 these clips are mounted on the wooden heads of the coil, while in the design shown in Fig. 107 they are mounted on the base, as is clearly indicated.

Repeating Coil. The so-called repeating coil used in telephony is really nothing but an induction coil. It is used in a variety of ways and usually has for its purpose the inductive association of two circuits that are conductively separated. Usually the repeating coil has a one to one ratio of turns, that is, there are the same number of turns in the primary as in the secondary. However, this is not always the case, since sometimes they are made to have an unequal number of turns, in which case they are called step-up or step-down  repeating coils, according to whether the primary has a smaller or a greater number of turns than the secondary. Repeating coils are almost universally of the closed magnetic circuit type.

Ringing and Talking Considerations. Since repeating coils often serve to connect two telephones, it follows that it is sometimes necessary to ring through them as well as talk through them. By this is meant that it is necessary that the coil shall be so designed as to be capable of transforming the heavy ringing currents as well as the very much smaller telephone or voice currents. Ringing currents ordinarily have a frequency ranging from about 16 to 75 cycles per second, while voice currents have frequencies ranging from a few hundred up to perhaps ten thousand per second. Ordinarily, therefore, the best form of repeating coil for transforming voice currents is not the best for transforming the heavy ringing currents and vice versâ. If the comparatively heavy ringing currents alone were to be considered, the repeating coil might well be of heavy construction with a large amount of iron in its magnetic circuit. On the other hand, for carrying voice currents alone it is usually made with a small amount of iron and with small windings, in order to prevent waste of energy in the core, and to give a high degree of responsiveness with the least amount of distortion of wave form, so that the voice currents will retain as far as possible their original characteristics. When, therefore, a coil is required to carry both ringing and talking currents, a compromise must be effected.

Types. The form of repeating coil largely used for both ringing and talking through is shown in Fig. 109. This coil comprises a soft iron core made up of a bundle of wires about .02 inch in diameter, the ends of which are left of sufficient length to be bent back around the windings after they are in place and thus form a completely closed magnetic path for the core. The windings of this particular coil are four in number, and contain about 2,400 turns each, and have a resistance of about 60 ohms. In this coil, when connected for local battery work, the windings are connected in pairs in series, thus forming effectively two windings having about 120 ohms resistance each. The whole coil is enclosed in a protecting case of iron. The terminals are brought out to suitable clips on the wooden base, as shown. An external perspective view of this coil is shown in Fig. 110. By bringing out each terminal of each winding, eight in all, as shown in this figure, great latitude of connection is provided for, since the windings may be connected in circuit in any desirable way, either by connecting them together in pairs to form virtually a primary and a secondary, or, as is frequently the case, to split the primary and the secondary, connecting a battery between each pair of windings.

Illustration: Fig. 109. Repeating Coil 
Fig. 109. Repeating Coil
View full size illustration.
Illustration: Fig. 110. Repeating Coil 
Fig. 110. Repeating Coil
View full size illustration.

Fig. 111 illustrates in section a commercial type of coil designed for talking through only. This coil is provided with four windings of 1,357 turns each, and when used for local battery work the coils are connected in pairs in series, thus giving a resistance of about 190 ohms in each half of the repeating coil. The core of this coil consists of a bundle of soft iron wires, and the shell which forms the return path for the magnetic lines is of very soft sheet iron. This shell is drawn into cup shape and its open end is closed, after the coil is inserted, by the insertion of a soft iron head, as indicated. As in the case of the coil shown in Figs. 109 and 110, eight terminals are brought out on this coil, thus providing the necessary flexibility of connection.

Illustration: Fig. 111. Repeating Coil 
Fig. 111. Repeating Coil
View full size illustration.
Illustration: Fig. 112. Diagram of Toroidal Repeating Coil 
Fig. 112. Diagram of Toroidal Repeating Coil
View full size illustration.
Illustration: Fig. 113. Toroidal Repeating Coil 
Fig. 113. Toroidal Repeating Coil
View full size illustration.

Still another type of repeating coil is illustrated in diagram in Fig. 112, and in view in Fig. 113. This coil, like the impedance coil shown in Fig. 104, comprises a core made up of a bundle of soft iron wires wound into the form of a ring. It is usually provided with two primary windings placed opposite each other upon the core, and with two secondary windings, one over each primary. In practice these two primary windings are connected in one circuit and the two secondaries in another. This is the standard repeating coil now used by the Bell companies in their common-battery cord circuits.

Illustration: Fig. 114. Symbol of Induction Coil 
Fig. 114. Symbol of Induction Coil
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Conventional Symbols . The ordinary symbol for the induction coil used in local battery work is shown in Fig. 114. This consists merely of a pair of parallel zig-zag lines. The primary winding is usually indicated by a heavy line having a fewer number of zig-zags, and the secondary by a finer line having a greater number of zig-zags. In this way the fact that the primary is of large wire and of comparatively few turns is indicated. This diagrammatic symbol may be modified to suit almost any conditions, and where a tertiary as well as a secondary winding is provided it may be shown by merely adding another zig-zag line.

Illustration: Fig. 115. Repeating-Coil Symbols 
Fig. 115. Repeating-Coil Symbols
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The repeating coil is indicated symbolically in the two diagrams of Fig. 115. Where there is no necessity for indicating the internal connections of the coil, the symbol shown in the left of this figure is usually employed. Where, however, the coil consists of four windings rather than two and the method of connecting them is to be indicated, the symbol at the right hand is employed. In Fig. 116 another way of indicating a four-winding repeating coil or induction coil is shown. Sometimes such windings may be combined by connection to form merely a primary and a secondary winding, and in other cases the four windings all act separately, in which case one may be considered the primary and the others, respectively, the secondary, tertiary, and quaternary.

Illustration: Fig. 116. Symbol of Four-Winding Repeating Coil 
Fig. 116. Symbol of Four-Winding Repeating Coil
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Where the toroidal type of repeating coil is employed, the diagram of Fig. 112, already referred to, is a good symbolic representation.