Cunynghame’s Ammeter

A modification of the Siemens' electro-dynamometer. (See Siemens' Electro-dynamometer) An electro-magnet with very massive core is excited by the current. As the core is of small reluctance the strength of the magnet is nearly proportional to the current strength. Between the poles of the magnet a soft iron armature or induced magnet is pivoted. It carries a pointer so adjusted that when the axis of the soft iron magnet is at an angle of about 30º with the line joiningthe poles of the electro-magnet the pointer will indicate zero.

The soft iron armature is so massive that the magnetism induced in it is proportional to the strength of the electro-magnet. Hence the couple exerted by the electro-magnet on the pivoted armature will be proportional to the square of the current.

The armature is retained in place by a spiral spring lying in line with its axis of rotation. The instrument is operated as a zero reading instrument. The current is passed through it. The needle is deflected; it is brought back to zero by turning a milled head which twists the spring. The current will be proportional to the square root of the angle of displacement of the milled head. A scale with index is provided, giving directly the square roots of the angle over which the pointer is moved.

The same instrument is wound for use as a volt-meter.