Direct current in a moving-coil meter deflects the coil in proportion to the amount of current.
A current meter is a low-resistance meter connected in series to read the amount of current in a circuit.
A meter shunt RS in parallel with the meter movement extends the range of a current meter [see Formula (8–1)].
A voltmeter consists of a meter movement in series with a high-resistance multiplier. The voltmeter with its multiplier is connected across two points to measure the potential difference in volts. The multiplier R can be calculated from Formula (8–3).
The ohms-per-volt rating of a voltmeter with series multipliers specifies the sensitivity on all voltage ranges. It equals the reciprocal of the full-scale deflection current of the meter. A typical value is 20,000 Ω/V for a voltmeter using a 50-μA movement. The higher the ohm-sper-volt rating, the better.
Voltmeter resistance RV is higher for higher ranges because of higher resistance multipliers. Multiply the ohms-per-volt rating by the voltage range to calculate the RV for each range.
An ohmmeter consists of an internal battery in series with the meter movement. Power must be off in the circuit being checked with an ohmmeter. The series ohmmeter has a back-off scale with zero ohms at the right edge and infinity at the left. Adjust for zero ohms with the leads short-circuited each time the ohms range is changed.
The VOM is a portable multimeter that measures volts, ohms, and milliamperes.
The digital multimeter generally has an input resistance of 10Ω on all dc voltage ranges.
In checking wire conductors, the ohmmeter reads 0 Ω OR very low R for normal continuity and infinite ohms for an open.
