Direct-Current Ammeter

Author: E.E. Kimberly

It is feasible to use the springs of a voltmeter element like that shown in Figs. 16-1 and Figs. 16-2 in meters intended to measure 5 amperes or less. Therefore, such an element without series resistance may be calibrated and used as an ammeter. In measuring direct currents greater than 5 amperes, a definite known fraction of the current is diverted around the meter, but the meter is calibrated in terms of the total current.

A voltmeter connected across a resistor measures the voltage drop, which is proportional to the current in the resistor. Hence, a voltmeter connected across a known unvarying resistance may be calibrated in amperes. Such a resistor is called a shunt. The voltmeter so used has an extremely small resistance and is called a millivoltmeter. The combination of a millivoltmeter and a shunt is called an ammeter.

For small currents the shunt is usually enclosed in the millivoltmeter case. For large currents the heat liberated by the shunt is too great to be tolerated in the meter case, and so the shunt is separately mounted. In switchboard installations the shunt is frequently placed several feet away from the meter. The meter must be calibrated with connecting leads having the same resistance as those to be used between it and the shunt, because the lead resistance is a large portion of the total millivoltmeter-circuit resistance and an indis criminate use of long or short leads may introduce intolerable errors in calibration. Fig. 16-4 shows proper connections of the two forms of d-c ammeters.

Fig. 16-4. Direct-Current Ammeter Connections

Alternating-Current Dynamometer-Type Voltmeters and Ammeters.

If a D'Arsonval-type direct-current voltmeter be connected to a source of alternating voltage, the torque is alternately in opposite directions and produces no readable deflection.

Fig. 16-5. Elements of Dynamometer Type Instrument

If, however, the constant field of the permanent magnet be replaced by the alternating field of a coil connected in series with the movable coil, then the fields of the two coils reverse simultaneously, the successive torque pulses are in the same direction, and a deflection of the pointer results. Such an instrument, the elements of which are shown in Fig. 16-5, is called a dynamometer-type meter. The dynamometer-type voltmeter may be used with alternating or direct voltage. The scale divisions are badly crowded in the lower part, and readings in the lowest 20 per cent of the scale are not reliable.

Ammeters of the dynamometer type are available only in low ranges because of the difficulty of carrying large current in the springs of the moving coil This type of meter measures effective values.