Ohmmeter is an electrical device which is used to measure the value of unknown resistance.

Conversion of galvanometer into ohmmeter

A galvanometer can be converted into ohmmeter by connecting an adjustable win resistance r and a cell in series with galvanometer.

How to measure an unknown resistance

Calibration

First of all, the terminals c and d are short circuited (i.e. R = 0) Adjust the resistance r such that the galvanometer gives full scale deflection. The full scale deflection is marked as 0 for resistance measurement. Now, the terminals c and A moving coil galvanometer is d are open circuited (i.e. R = 0). No current passes through galvanometer and converted into an ohmmeter. thus the deflection is zero. Zero deflection is marked as infinity for resistance measurement.

Next, a known resistance R is connected between terminais c and d. The galvanometer gives deflection to some intermediate point. This point is calibrated as R. In this way, the whole scale is calibrated into resistance.

Unknown resistance

Now, to measure an unknown resistance, it is connected between the terminals c and d. The deflection on the calibrated scale gives the value of resistance directly.