Conversion of galvanometer to ammeter and voltmeter - explanation, sensitivity of voltmeter and ammeter, practice problems, FAQs.

You went to the physics lab, your teacher instructed you to perform a simple electrical experiment: take a battery, some electrical wires, connect them to a bulb. But there is a problem: the bulb will fuse if huge currents pass through it, so you need to measure the amount of current flowing in the wires. A device known as ammeter is going to help us achieve this. In order to spice up the experiment, you connect a resistor to the same circuit. Now current flows through the resistor as well, but you do not want to risk overloading the resistor. In other words, you need control over how much voltage can be applied to the resistor. An equipment known as voltmeter is going to help us find this. A galvanometer is a device which can be used both as a voltmeter and ammeter; however unlike an ammeter, a galvanometer measures only minute currents passing through it. In this article, we will explore how to convert a galvanometer to an ammeter and a voltmeter. 

Table of contents

• What is a galvanometer?
• What is an ammeter? 
• Converting a galvanometer into an ammeter
• What is a voltmeter?
• Converting a galvanometer into a voltmeter
• Current sensitivity
• Voltage sensitivity
• Practice problems
• FAQs

What is a galvanometer?

A galvanometer is a device that can detect the presence of small currents and measure them; since large currents will damage the coil. A needle shows deflection on a scale graduated in ampere (unit of current). The maximum current that can cause full scale deflection in a galvanometer is called “ maximum deflection current The value of is of the order of

                  Galvanometer

What is an ammeter?

An ammeter is an equipment used to measure large currents. An ideal ammeter has zero resistance. Lesser the resistance of the ammeter, more accurate its reading will be. Ammeter is always connected in series in a circuit. 

Converting a galvanometer into an ammeter

A galvanometer can be converted into an ammeter by connecting a low resistance called shunt in parallel with it; so that the majority of the current will be allowed to pass through the shunt while only a small portion of the current will pass through the galvanometer. 

Let be the current in the circuit and be the full scale deflection of the galvanometer.

is the current that will pass through the shunt. The combination of galvanometer and shunt will behave as an ammeter. Let be the value of shunt resistance and indicate the galvanometer resistance. Since they are connected in parallel, their voltage drop would be the same, meaning the product of their respective currents and resistances are equal.

I.e,  

The above equation gives the value of shunt resistance which needs to be connected in parallel with the galvanometer to be able to measure current. 

The effective resistance of the ammeter so formed is given by .

What is a voltmeter?

Voltmeter is a device which is used to measure the voltage or potential difference that has occurred across an appliance. Voltmeter is always connected in parallel. 

An ideal voltmeter is one which has infinite resistance. 

Converting a galvanometer into a voltmeter

A galvanometer is converted into a voltmeter by connecting a high resistance in series with it. In the following diagram, let indicate the resistance of the galvanometer. The resistor to be used in series is Let be the voltage across the combination, then by Ohm’s law, 

The above equation gives an idea about the resistance that needs to be connected in series.

Current sensitivity

The current sensitivity of a galvanometer is defined as the deflection produced in it when a unit current is passed through it. Let be the deflection produced in the galvanometer when a current of ampere is passed through it. Then current sensitivity

The unit of current sensitivity is (divisions/milliampere). It does not depend upon the value of galvanometer resistance

Voltage sensitivity

The deflection produced in the galvanometer for a unit voltage across it is called voltage sensitivity. Let be the deflection produced in the galvanometer when a voltage of is applied across its coil. Then voltage sensitivity. voltage sensitivity depends upon the galvanometer resistance  

Practice problems

Q. Construct a voltmeter from a galvanometer whose resistance is given by and a full scale deflection of into a voltmeter with a maximum range of ? 

A.

 Given , galvanometer resistance ,  

 Let be the resistance that needs to be connected in series. Then, 

Q. Calculate the value of shunt resistance required to convert a galvanometer into an ammeter with a range of to

(a) (b) ( c ) (d)

A. a
Given, full scale deflection of the galvanometer,

The value of shunt resistance that needs to be connected is  

Solving, we get

Q. Calculate the shunt to be connected in parallel with a galvanometer if of the main current should be allowed to pass through the galvanometer. Given, the resistance of the galvanometer is  

(a) (b) (c) (d)

Solution) c

Let indicate the main current and indicate the galvanometer current. 

Given

Q. A galvanometer of internal resistance requires for a full scale deflection. To convert this into a voltmeter that shows full scale deflection of we need to connect a resistance of 

(a) in series with it (b) in series with it 

(c) in series with it (d) in parallel with it

A. b

Given

 Substituting the values, we get

To convert a galvanometer into a voltmeter, we need to connect a high resistance in series.

FAQs

Q. Write the expression for the effective resistance of the combination having resistance that is connected in series with a galvanometer of resistance
A. Since they are connected in series, effective resistance of the combination sum of the resistance

Q. What is the difference between an ammeter and galvanometer?
A. A galvanometer is used to measure small currents of the order of milliampere while an ammeter can measure large currents.

Q. An ideal ammeter has zero resistance. Why?
A. An ideal ammeter has zero resistance because it allows current to flow through it completely. 

Q. An ideal voltmeter has infinite resistance. Why?
A. Ideally there should be no current through the voltmeter and the entire current must pass through the element across which voltage is to be measured, so that we get the actual value of voltage. So, an ideal voltmeter has infinite resistance.