To measure resistance by the ammeter-voltmeter method
For this method we require a voltmeter of very high resistance and an ammeter of very low resistance. The unknown resistor, R, is connected in series with an ammeter, a rheostat, a plug key or other switch, and one or more cells. The voltmeter is connected across the ends of R (Fig. 40.4 (a)). As a safety precaution to avoid damage to the ammeter.
through overload, the rheostat should be set to maximum resistance, before closing the circuit. The rheostat is then altered to give a series of suitable ammeter and voltmeter readings.
The results may be entered in a table as below and a mean value for R calculated