9.19 Use of a Galvanometer

9.19 Use of a Galvanometer

A galvanometer is an instrument for detecting a current. We are not going to discuss its internal structure and how does it work. We focus only on its use. It is often used in null methods to achieve precise measurements in electrical circuits. The null method involves adjusting the circuit until the galvanometer shows no deflection i.e., a zero reading. This indicates that certain required conditions are met in the circuit. In this state, the electric potentials at both ends of the galvanometer are the same. Although a galvanometer has its own resistance, but at the null reading, its resistance does not come into play. The reason is that, in this condition no current is passing through it. The null method is widely used in bridge circuits such as Wheatstone and potentiometer setups.

As we have studied in the previous section, the null method is used to measure an unknown resistance in the Wheatstone bridge circuits. The galvanometer is connected between the mid-points of opposite sides. The variable resistance is adjusted until the galvanometer shows no deflection. At this point, the bridge is balanced and the unknown resistance can be calculated using the ratio of the known resistances.

In a potentiometer, null method is used to measure an unknown voltage by comparison with a known reference voltage applied across the resistance wire of the potentiometer. A galvanometer and a jockey are used to make contact along the wire. At null point, the potential difference between the jockey and the end of the wire equals the unknown voltage. The position of the jockey gives the measure of the unknown voltage.

There are some advantages of using a galvanometer in null method:

1. Null method eliminates the effect of the galvanometer's internal resistance on the measurement resulting in more accurate readings.
2. Galvanometers are highly sensitive and can detect very small currents of the order of $10^{-8}$ ampere.
3. "No deflection" indicates a direct and clear condition of balance making it easier to identify the null point.