Conversion of Galvanometer into a Ammeter

The conversion of a galvanometer into an ammeter is a laboratory procedure used to enable a sensitive current-measuring device (galvanometer) to measure larger currents reliably.

Description

Principle: A galvanometer can only measure small currents directly. To convert it into an ammeter capable of measuring higher currents, a low-value shunt resistor is connected in parallel with the galvanometer.

Apparatus:

Sensitive galvanometer,

Precision shunt resistor,

Connecting terminals and wires,

Calibrated scale or dial.

Operation: When placed in a circuit, most of the current bypasses the galvanometer through the shunt resistor, so the galvanometer only measures a small fraction. The scale is recalibrated to represent the total current flowing through the meter.

Formula: The value of the shunt resistor (

S

S) required is

S

=

I

g

⋅

G

I

−

I

g

S=

I−I

g

I

g

⋅G

where

I

I is the desired full-scale current,

I

g

I

g

is the galvanometer’s full-scale current, and

G

G is the galvanometer’s resistance.