ABSTRACT
Meters Straightforward measurements of voltage, current and resistance can provide useful information on the state of almost any circuit. To get the best from a meter it is not only necessary to select an appropriate measurement function and range but also to be aware of the limitations of the instrument
and the effect that it might have on the circuit under investigation. When fault finding, it is interpretation that is put on the meter readings rather than the indications themselves. Figures 14.2(a) and 14.2(b) respectively show the circuit of a simple voltmeter and a simple ammeter. Each instrument is based on the moving coil indicator shown in Fig. 14.1. The voltmeter consists of a multiplier resistor connected in series with the basic moving coil movement whilst the ammeter consists of a shunt resistor connected in parallel with the basic moving coil instrument. When determining the value of multiplier or shunt resistance (Rm and Rs respectively in Fig. 14.2) it is important to remember that the coil of the moving coil meter also has a resistance. We have shown this as a resistor, r, connected with the moving coil in Fig. 14.3. In both cases, the current required to produce full-scale deflection of the meter movement is Im.
