A. Large magnet. B. Soft-iron keeper magnetized by magnet and acting as resistance. D. Cylinder turning within B, and actuated by current entering at C1, and flowing through spiral wire (not shown) at base of D, and through coil on cylinder to terminal C2. E. Hair-spring regulating pointer. F. Pointer stops.
Am´meter (short for ampere-meter), an instrument used for the measurement of electric currents. For commercial use the scale is marked so as to read amperes directly, but for experimental purposes it is usual to have a scale with divisions numbered in tens, in which case the reading multiplied by a suitable constant gives the value of the current in amperes. By employing suitable shunts this admits of the one instrument being used for a number of ranges.
The types of ammeter and the principles upon which they work are as follows: (a) Soft-iron type, the action of a magnetic field on a piece of soft iron; (b) moving-coil type and dynamometer type, the action of a magnetic field on a current-carrying coil; (c) hot-wire type, the expansion of a conductor due to the heating produced by the current; (d) induction type, the action of a magnetic field on the eddy currents produced in a metal disc.
The "soft-iron" ammeter can be used for both direct and alternating currents, is inexpensive, and is sufficiently accurate for commercial use.
For direct-current measurements where a high degree of accuracy is of first importance, a "moving-coil" ammeter is invariably used.
In alternating-current circuits its place is taken by the dynamometer type, which reads both direct and alternating currents.
In cases where absence of inductance in the instrument is important, e.g. in the measurements in wireless-telegraph and telephone circuits, the "hot-wire" ammeter is used. It measures both direct and alternating currents, and, when properly used, has a high degree of accuracy.
The "induction" type cannot be used for direct currents, and has the limitation that with alternating currents it will read correctly only at the frequency for which it is calibrated.
Almost invariably an ammeter gives its full-scale reading when a small current, say of the order of one-tenth of an ampere, is passing through the instrument itself. In order to read larger currents a device is employed whereby a definite fraction of the current to be measured
passes through the instrument.—Bibliography: J. A. Fleming, A Handbook for the Electrical Laboratory and Testing Room (2 vols.); G. D. Aspinall Parr, Electrical Measuring Instruments.