20. Internal Resistance of a Meter

The internal resistance of the Leybold multimeter on the 10 V scale can be measured with the circuit below:

The ammeter reads the entire current flowing through the internal resistance of the voltmeter, and the voltmeter reads the voltage drop across this resistance, so ri = V/i. You can check this by inserting a resistance decade box in series with the circuit and adjusting the resistance until the meter readings go to one half their former values. The decade box will then read the internal resistance of the voltmeter (assuming that of the ammeter is much smaller).

One way to measure the internal resistance of an ammeter is the circuit below.

The decade box R2 is set to zero, and R1 is chosen for an approximate full scale reading for the ammeter. R2 is then adjusted to halve this reading, so its resistance equals that of the ammeter.

ri for Leybold multimeter on 10 V scale = 34,000 ohms

ri for Leybold multimeter on 1 mA scale = 120 ohms

ri for Leybold projection meter on 1 mA scale = 100 ohms