Question

In half deflection method, a high resistance box is connected in series with the battery so that
A. The deflection of the galvanometer is brought within the scale
B. Power losses are minimized.
C. High resistance values are easily available.
D. None of the above.

Answer 1

Hint: The role of high resistance connected in series is to reduce the value of current in the circuit. Galvanometer will show full deflection when very large current flows through it.

Complete Step-by-Step solution:
Half deflection method:
The half deflection method is used to find out the resistance of a galvanometer.
The galvanometer having resistance is connected with resistance R and battery B. The shunt resistance is connected in parallel with the galvanometer. The key K2 is closed and current flows through the circuit and the galvanometer shows deflection. This deflection in a galvanometer is given by
${{K}_{\theta }}=\dfrac{E}{(R+G)}$
So the value of G is usually in ohms. So if the value of R is also small then deflection could be large. Hence to bring deflections within scale a high resistance is used in series with a battery.
When the key K2 is also closed the galvanometer and shunt resistance will be in and parallel and net resistance will be reduced. This time the deflection shown in the galvanometer will reduce to half. Thus this method is known as the half deflection method.
Correct option is A. The deflection of the galvanometer is brought within the scale.

Note: The value of series resistance must be very larger than the shunt resistance S. In the absence of resistance R, since the galvanometer has small resistance value, the current flowing through the circuit will be very high and full deflection may be shown in the galvanometer. To avoid damage to the galvanometer we use high resistance in series with it.