Ammeter

AMMETER, an instrument used for measuring the strength of electric current. There are several systems of ammeters: 1) the Depré-d'Arsonval system, 2) ammeters "with soft iron", 3) thermal ammeters, 4) the Ferraris system and several others. The first of these listed systems is used only in direct current circuits, the second and third with equal success in both direct and alternating current circuits, and finally the fourth exclusively in alternating current circuits. For precise measurements, the Depré-d'Arsonval ammeters are the best. The current being measured or, more often, a small part of it, passes through turns of thin insulated wire wound onto a light frame located in the field of a permanent magnet. The stronger the current, the greater the angle at which the frame deflects, twisting a thin spiral spring. - In the ammeter with soft iron, the moving part is a piece of soft iron of one form or another, which is repelled from another such piece when both are magnetized by the effect of the current passing through the coil surrounding them. Unlike Depré instruments, the scale divisions here are uneven, and the accuracy of readings varies with different current strengths: the first divisions of the scale are usually unreliable. - Thermal ammeters are very simple in construction. A portion of the current being measured passes here through a thin thread and heats it. When heated, the thread lengthens, and since its ends are fixed immovably, its middle bends somewhat. The displacement of the thread is transmitted by means of other, intermediate, threads to a pulley which sits on the axis of a pointer moving along the scale.

Ammeter of the drum type.

- Instruments of the Ferraris (or Ferrari) system are based on the action of a rotating magnetic field created by alternating current. In such a field, a light drum is placed here, which tends to rotate but is held back by a spiral spring. With the same ammeter, one can measure the strength of currents from fractions of an ampere to thousands of amperes: it is only necessary to direct a certain definite part of the entire current into the instrument, which may be very small. For this purpose, so-called shunts (for direct current) and current transformers (for alternating current) are used.

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