Hook switch

The Hook Switch

Purpose. In complete telephone instruments, comprising both talking and signaling apparatus, it is obviously desirable that the two sets of apparatus, for talking and signaling respectively, shall not be connected with the line at the same time. A certain switching device is, therefore, necessary in order that the signaling apparatus alone may be left operatively connected with the line while the instrument is not being used in the transmission of speech, and in order that the signaling apparatus may be cut out when the talking apparatus is brought into play.

In instruments employing batteries for the supply of transmitter current, another switching function is the closing of the battery circuit through the transmitter and the induction coil when the instrument is in use for talking, since to leave the battery circuit closed all the time would be an obvious waste of battery energy.

In the early forms of telephones these switching operations were performed by a manually operated switch, the position of which the user was obliged to change before and after each use of the telephone. The objection to this was not so much in the manual labor imposed on the user as in the tax on his memory. It was found to be practically a necessity to make this switching function automatic, principally because of the liability of the user to forget to move the switch to the proper position after using the telephone, resulting not only in the rapid waste of the battery elements but also in the inoperative condition of the signal-receiving bell. The solution of this problem, a vexing one at first, was found in the so-called automatic hook switch or switch hook, by which the circuits of the instrument were made automatically to assume their proper conditions by the mere act, on the part of the user, of removing the receiver from, or placing it upon, a conveniently arranged hook or fork projecting from the side of the telephone casing.

Automatic Operation. It may be taken as a fundamental principle in the design of any piece of telephone apparatus that is to be generally used by the public, that the necessary acts which a person must perform in order to use the device must, as far as possible, follow as a natural result from some other act which it is perfectly obvious to the user that he must perform. So in the case of the switch hook, the user of a telephone knows that he must take the receiver from its normal support and hold it to his ear; and likewise, when he is through with it, that he must dispose of it by hanging it upon a support obviously provided for that purpose.

In its usual form a forked hook is provided for supporting the receiver in a convenient place. This hook is at the free end of a pivoted lever, which is normally pressed upward by a spring when the receiver is not supported on it. When, however, the receiver is supported on it, the lever is depressed by its weight. The motion of the lever is mechanically imparted to the members of the switch proper, the contacts of which are usually enclosed so as to be out of reach of the user. This switch is so arranged that when the hook is depressed the circuits are held in such condition that the talking apparatus will be cut out, the battery circuit opened, and the signaling apparatus connected with the line. On the other hand, when the hook is in its raised position, the signaling apparatus is cut out, the talking apparatus switched into proper working relation with the line, and the battery circuit closed through the transmitter.

In the so-called common-battery telephones, where no magneto generator or local battery is included in the equipment at the subscriber's station, the mere raising of the hook serves another important function. It acts, not only to complete the circuit through the substation talking apparatus, but, by virtue of the closure of the line circuit, permits a current to flow over the line from the central-office battery which energizes a signal associated with the line at the central office. This use of the hook switch in the case of the common-battery telephone is a good illustration of the principle just laid down as to making all the functions which the subscriber has to perform depend, as far as possible, on acts which his common sense alone tells him he must do. Thus, in the common-battery telephone the subscriber has only to place the receiver at his ear and ask for what he wants. This operation automatically displays a signal at the central office and he does nothing further until the operator inquires for the number that he wants. He has then nothing to do but wait until the called-for party responds, and after the conversation his own personal convenience demands that he shall dispose of the receiver in some way, so he hangs it up on the most convenient object, the hook switch, and thereby not only places the apparatus at his telephone in proper condition to receive another call, but also conveys to the central office the signal for disconnection.

Likewise in the case of telephones operating in connection with automatic exchanges, the hook switch performs a number of functions automatically, of which the subscriber has no conception; and while, in automatic telephones, there are more acts required of the user than in the manual, yet a study of these acts will show that they all follow in a way naturally suggested to the user, so that he need have but the barest fundamental knowledge in order to properly make use of the instrument. In all cases, in properly designed apparatus, the arrangement is such that the failure of the subscriber to do a certain required act will do no damage to the apparatus or to the system, and, therefore, will inconvenience only himself.

Design. The hook switch is in reality a two-position switch, and while at present it is a simple affair, yet its development to its high state of perfection has been slow, and its imperfections in the past have been the cause of much annoyance.

Several important points must be borne in mind in the design of the hook switch. The spring provided to lift the hook must be sufficiently strong to accomplish this purpose and yet must not be strong enough to prevent the weight of the receiver from moving the switch to its other position. The movement of this spring must be somewhat limited in order that it will not break when used a great many times, and also it must be of such material and shape that it will not lose its elasticity with use. The shape and material of the restoring spring are, of course, determined to a considerable extent by the length of the lever arm which acts on the spring, and on the space which is available for the spring.

The various contacts by which the circuit changes are brought about upon the movement of the hook-switch lever usually take the form of springs of German silver or phosphor-bronze, hard rolled so as to have the necessary resiliency, and these are usually tipped with platinum at the points of contact so as to assure the necessary character of surface at the points where the electric circuits are made or broken. A slight sliding movement between each pair of contacts as they are brought together is considered desirable, in that it tends to rub off any dirt that may have accumulated, yet this sliding movement should not be great, as the surfaces will then cut each other and, therefore, reduce the life of the switch.

Contact Material. On account of the high cost of platinum, much experimental work has been done to find a substitute metal suitable for the contact points in hook switches and similar uses in the manufacture of telephone apparatus. Platinum is unquestionably the best known material, on account of its non-corrosive and heat-resisting qualities. Hard silver is the next best and is found in some first-class apparatus. The various cheap alloys intended as substitutes for platinum or silver in contact points may be dismissed as worthless, so far as the writers' somewhat extensive investigations have shown.

In the more recent forms of hook switches, the switch lever itself does not form a part of the electrical circuit, but serves merely as the means by which the springs that are concerned in the switching functions are moved into their alternate cooperative relations. One advantage in thus insulating the switch lever from the current-carrying portions of the apparatus and circuits is that, since it necessarily projects from the box or cabinet, it is thus liable to come in contact with the person of the user. By insulating it, all liability of the user receiving shocks by contact with it is eliminated.

Wall Telephone Hooks. Kellogg. A typical form of hook switch, as employed in the ordinary wall telephone sets, is shown in Fig. 83, this being the standard hook of the Kellogg Switchboard and Supply Company. In this the lever 1  is pivoted at the point 3  in a bracket 5  that forms the base of all the working parts and the means of securing the entire hook switch to the box or framework of the telephone. This switch lever is normally pressed upward by a spring 2, mounted on the bracket 5, and engaging the under side of the hook lever at the point 4. Attached to the lever arm 1  is an insulated pin 6. The contact springs by which the various electrical circuits are made and broken are shown at 78910, and 11, these being mounted in one group with insulated bushings between them; the entire group is secured by machine screws to a lug projecting horizontally from the bracket 5. The center spring 9  is provided with a forked extension which embraces the pin 6  on the hook lever. It is obvious that an up-and-down motion of the hook lever will move the long spring 9  in such manner as to cause electrical contact either between it and the two upper springs 7  and 8, or between it and the two lower springs 10  and 11. The hook is shown in its raised position, which is the position required for talking. When lowered the two springs 7  and 8  are disengaged from the long spring 9  and from each other, and the three springs 910, and 11  are brought into electrical engagement, thus establishing the necessary signaling conditions.

Illustration: Fig. 83. Long Lever Hook Switch 
Fig. 83. Long Lever Hook Switch
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The right-hand ends of the contact springs are shown projecting beyond the insulating supports. This is for the purpose of facilitating making electrical joints between these springs and the various wires which lead from them. These projecting ends are commonly referred to as ears, and are usually provided with holes or notches into which the connecting wire is fastened by soldering.

Western Electric. Fig. 84 shows the type of hook switch quite extensively employed by the Western Electric Company in wall telephone sets where the space is somewhat limited and a compact arrangement is desired. It will readily be seen that the principle on which this hook switch operates is similar to that employed in Fig. 83, although the mechanical arrangement of the parts differs radically. The hook lever 1  is pivoted at 3 on a bracket 2, which serves to support all the other parts of the switch. The contact springs are shown at 45, and 6, and this latter spring 6  is so designed as to make it serve as an actuating spring for the hook. This is accomplished by having the curved end of this spring press against the lug 7 of the hook and thus tend to raise the hook when it is relieved of the weight of the receiver. The two shorter springs 8  and 9  have no electrical function but merely serve as supports against which the springs 4  and 5  may rest, when the receiver is on the hook, these springs 4  and 5  being given a light normal tension toward the stop springs 8  and 9. It is obvious that in the particular arrangement of the springs in this switch no contacts are closed when the receiver is on the hook.

Illustration: Fig. 84. Short Lever Hook Switch 
Fig. 84. Short Lever Hook Switch
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Concerning this latter feature, it will be noted that the particular form of Kellogg hook switch, shown in Fig. 83, makes two contacts and breaks two when it is raised. Similarly the Western Electric Company's makes two contacts but does not break any when raised. From such considerations it is customary to speak of a hook such as that shown in Fig. 83 as having two make and two break contacts, and such a hook as that shown in Fig. 84 as having two make contacts.

It will be seen from either of these switches that the modification of the spring arrangement, so as to make them include a varying number of make-and-break contacts, is a simple matter, and switches of almost any type are readily modified in this respect.

Illustration: Fig. 85. Removable Lever Hook Switch 
Fig. 85. Removable Lever Hook Switch
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Dean. In Fig. 85 is shown a decidedly unique hook switch for wall telephone sets which forms the standard equipment of the Dean Electric Company. The hook lever 1  is pivoted at 2, an auxiliary lever 3  also being pivoted at the same point. The auxiliary lever 3  carries at its rear end a slotted lug 4, which engages the long contact spring 5, and serves to move it up and down so as to engage and disengage the spring 6, these two springs being mounted on a base lug extending from the base plate 7, upon which the entire hook-switch mechanism is mounted. The curved spring 8, also mounted on this same base, engages the auxiliary lever 3  at the point 9  and normally serves to press this up so as to maintain the contact springs 5  in engagement with contact spring 6. The switch springs are moved entirely by the auxiliary lever 3, but in order that this lever 3  may be moved as required by the hook lever 1, this lever is provided with a notched lug 10  on its lower side, which notch is engaged by a forwardly projecting lug 11  that is integral with the auxiliary lever 3. The switch lever may be bodily removed from the remaining parts of the hook switch by depressing the lug 11  with the finger, so that it disengages the notch in lug 10, and then drawing the hook lever out of engagement with the pivot stud 2, as shown in the lower portion of the figure. It will be noted that the pivotal end of the hook lever is made with a slot instead of a hole as is the customary practice.

The advantage of being able to remove the hook switch bodily from the other portions arises mainly in connection with the shipment or transportation of instruments. The projecting hooks cause the instruments to take up more room and thus make larger packing boxes necessary than would otherwise be used. Moreover, in handling the telephones in store houses or transporting them to the places where they are to be used, the projecting hook switch is particularly liable to become damaged. It is for convenience under such conditions that the Dean hook switch is made so that the switch lever may be removed bodily and placed, for instance, inside the telephone box for transportation.

Desk-Stand Hooks. The problem of hook-switch design for portable desk telephones, while presenting the same general characteristics, differs in the details of construction on account of the necessarily restricted space available for the switch contacts in the desk telephone.

Western Electric. In Fig. 86 is shown an excellent example of hook-switch design as applied to the requirements of the ordinary portable desk set. This figure is a cross-sectional view of the base and standard of a familiar type of desk telephone. The base itself is of stamped metal construction, as indicated, and the standard which supports the transmitter and the switch hook for the receiver is composed of a black enameled or nickel-plated brass tube 1, attached to the base by a screw-threaded joint, as shown. The switch lever 2  is pivoted at 3  in a brass plug 4, closing the upper end of the tube forming the standard. This brass plug supports also the transmitter, which is not shown in this figure. Attached to the plug 4 by the screw 5  is a heavy strip 6, which reaches down through the tube to the base plate of the standard and is held therein by a screw 7. The plug 4, carrying with it the switch-hook lever 2  and the brass strip 6, may be lifted bodily out of the standard 1  by taking out the screw 7  which holds the strip 6  in place, as is clearly indicated. On the strip 6  there is mounted the group of switch springs by which the circuit changes of the instrument are brought about when the hook is raised or lowered. The spring 8  is longer than the others, and projects upwardly far enough to engage the lug on the switch-hook lever 2. This spring, which is so bent as to close the contacts at the right when not prevented by the switch lever, also serves as an actuating spring to raise the lever 2  when the receiver is removed from it. This spring, when the receiver is removed from the hook, engages the two springs at the right, as shown, or when the receiver is placed on the hook, breaks contact with the two right-hand springs and makes contact respectively with the left-hand spring and also with the contact 9  which forms the transmitter terminal.

Illustration: Fig. 86. Desk-Stand Lever Hook Switch 
Fig. 86. Desk-Stand Hook Switch
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It is seen from an inspection of this switch hook that it has two make and two break contacts. The various contact springs are connected with the several binding posts shown, these forming the connectors for the flexible cord conductors leading into the base and up through the standard of the desk stand. By means of the conductors in this cord the circuits are led to the other parts of the instrument, such as the induction coil, call bell, and generator, if there is one, which, in the case of the Western Electric Company's desk set, are all mounted separately from the portable desk stand proper.

This hook switch is accessible in an easy manner and yet not subject to the tampering of idle or mischievous persons. By taking out the screw 7 the entire hook switch may be lifted out of the tube forming the standard, the cords leading to the various binding posts being slid along through the tube. By this means the connections to the hook switch, as well as the contact of the switch itself, are readily inspected or repaired by those whose duty it is to perform such operations.

Kellogg. In Fig. 87 is shown a sectional view of the desk-stand hook switch of the Kellogg Switchboard and Supply Company. In this it will be seen that instead of placing the switch-hook springs within the standard or tube, as in the case of the Western Electric Company, they are mounted in the base where they are readily accessible by merely taking off the base plate from the bottom of the stand. The hook lever operates on the long spring of the group of switch springs by means of a toggle joint in an obvious manner. This switch spring itself serves by its own strength to raise the hook lever when released from the weight of the receiver.

Illustration: Fig. 87. Desk-Stand Lever Hook Switch 
Fig. 87. Desk-Stand Hook Switch
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In this switch, the hook lever, and in fact the entire exposed metal portions of the instrument, are insulated from all of the contact springs and, therefore, there is little liability of shocks on the part of the person using the instrument.

Conventional Symbols. The hook switch plays a very important part in the operation of telephone circuits; for this reason readily understood conventional symbols, by which they may be conveniently represented in drawings of circuits, are desirable. In Fig. 88 are shown several symbols such as would apply to almost any circuit, regardless of the actual mechanical details of the particular hook switch which happened to be employed. Thus diagram A  in Fig. 88 shows a hook switch having a single make contact and this diagram might be used to refer to the hook switch of the Dean Electric Company shown in Fig. 85, in which only a single contact is made when the receiver is removed, and none is made when it is on the hook. Similarly, diagram B  might be used to represent the hook switch of the Kellogg Company, shown in Fig. 83, the arrangement being for two make and two break contacts. Likewise diagram C  might be used to represent the hook switch of the Western Electric Company, shown in Fig. 84, which, as before stated, has two make contacts only. Diagram D  shows another modification in which contacts made by the hook switch, when the receiver is removed, control two separate circuits. Assuming that the solid black portion represents insulation, it is obvious that the contacts are divided into two groups, one insulated from the other.

Illustration: Fig. 88. Hook Switch Symbols 
Fig. 88. Hook Switch Symbols
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