Question

It is necessary to measure the resistance of a circuit operating at 120 V. There is available only one galvanometer with a sensitivity of 10^-5 A per division. How should the galvanometer be connected to work as an ohmmeter? What is the smallest resistance of the circuit that can be measured by such a galvanometer if the whole scale has 40 divisions? Calibrate the scale of such an ohmmeter in ohms per division. The internal resistance of the instrument may be neglected.

Answer 1

The galvanometer should be connected in series with the circuit. The scale of the instrument will be: ∞, 1.2 x 10⁷ ohms, 6 x10⁶ ohms, 4 x 10⁶ ohms, (120/n) x 10⁵ ohms, where n is the number of the division. The smallest resistance which may be measured is 3 x 10⁵ ohms.

The value of the resistance Rn corresponding to the individual scale divisions of the galvanometer is determined from the formula

R_n = V/nI₀,

where V is the operating voltage, n is the number of the scale division, and I₀ is the current corresponding to one scale division of the galvanometer.