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Difference Between Ammeter and Voltmeter

August 2, 2022

The main difference between Ammeter and Voltmeter is the combination of resistors which is connected with a galvanometer to convert it into an ammeter and voltmeter.

Table of Contents

Ammeter

An ammeter is an electrical instrument that is used to measure electric current in amperes. It is basically a modified form of the galvanometer. The portion of the galvanometer whose motion causes the needle of the device to move across the scale is usually known as “Meter Movement”.

Most of the galvanometers give full-scale deflection when a few milliamperes current is passed through them. So an ordinary galvanometer cannot be used for measuring large currents without proper modification.

Conversion of galvanometer to ammeter

Consider a galvanometer, whose meter-movement has resistance R_g. When I_g, current passes through it, it gives full-scale deflection. If V_g is the electric potential difference across the Galvanometer then according to Ohm’s law

V_g= I_g R_g

In order to convert this galvanometer into an ammeter, we connect a small resistance R_s called a shunt or shunt resistance in parallel with the galvanometer.

This low resistance provides an additional path to the flow of current. The value of shunt resistance R_s is so adjusted that only I_g, current flows through the galvanometer so that it gives full-scale deflection.

The remaining current I_s=I-I_g passes through R_s.

The potential difference across shunt resistance R_s according to Ohm’s law is given by

V_s =R_sI_s

V_s =R_s(I-I_g)

As the shunt resistance, Rs, and Galvanometer resistance Rg are connected in parallel so the potential difference across both is the same. So

V_s=V_g

R_sI_s= I_g R_g

(I-I_g)R_s=I_gR_g

R_s= I_gR_g/ (I-I_g)

The resistance of the shunt is usually very small even a piece of copper can serve this purpose. Thus an ammeter is a low resistance Galvanometer and is always connected in series with a circuit.

The resistance of the ammeter is the combined resistance of the galvanometer meter movement and the shunt. Usually, it is very small.

An ammeter must have very low resistance so that it does not disturb the circuit in which it is connected in series in order to measure the current.

Voltmeter

An electrical device that is used to measure the potential difference in volts between two points is called a Voltmeter.

It is a modified form of the galvanometer and is always connected in parallel with the circuit. When a high resistance R_h is connected in series with the galvanometer, it becomes a voltmeter.

Suppose a galvanometer has resistance R_g and it gives full-scale deflection when I_g current passes through it.

Conversion of galvanometer to voltmeter

To convert this galvanometer into a voltmeter having a range of V volts, a resistor of high resistance R_h is connected in series with the coil of the galvanometer.

The value of R_h is so adjusted that I_g current must pass through it, so that meter movement shows full-scale deflection.

Since R_hand R_g are in series so the combined resistance is R_e= R_h +R_{g .}

According to Ohm’s law.

V=I_g R_e

V=I_g(R_h+R_g)

V/ I_g= R_h+R_g

R_g-(V/ I_g)= R_h

The scale of the galvanometer is calibrated from 0 to V volts. By properly choosing the value of the resistance R_h any voltage can be measured.

The voltmeter is always connected in parallel between two points in a circuit whose potential difference is to be measured. Before connecting a voltmeter, it should be assured that its resistance is very high in comparison with the resistance of the circuit across which it is connected otherwise it will load the circuit and will alter the potential difference which is required to be measured.