Problem 3: The coil of galvanometer which has a resistance of 50 Ohms and a current of 500 micro amperes produces full-scale deflection on it. How it can be converted into (a) an ammeter of 5 ampere range and (b) a volt meter of 300 volt range?

Solution:

Step: 1 Overview

Conversion of Galvanometer into an Volt meter:
A galvanometer can be converted into a volt meter by connecting a high resistance in series with it. This series resistance is denoted by R_x.

In the ammeter we altered the range by changing the value of shunt resistance, here in volt meter we can change the range of volt meter by changing the value of series resistance.
The equation of by which we can calculate how much resistance we needed for a particular range of volt meter is given below:

R_x=(V/I_g)-R_g

Step: 2 Calculation

Given:

R_g=50 Ohms
I_g=500 micro amperes = 0.0005A
I=5A
V=300V

Required:

R_s=?
R_x=?

We know that:

R_s= (I_g/(I-I_g))R_g
R_s= (0.0005/(5-0.0005))(50)
R_s= (0.0005/4.9995)(50)
R_s= 0.005 Ohms (Ans)

Also we know that:

R_x=(V/I_g)-R_g
R_x=(300/0.0005)-50
R_x=599950 Ohms (Ans)