Ammeter-Construction, Working principle, Practice problems,FAQs

Sarvesh buys two bulbs from the hardware store. He takes them to his physics lab, connects them in series to a power source. He notices that one of the bulbs heats up more in comparison to the other. He starts to wonder why this is happening; he then recalls his physics teacher telling him that the heat dissipated by a bulb varies as the square of the current flowing through it. So, if he has knowledge of the current flowing through each bulb, he can find out which one heats up faster. Not only does the current gives knowledge of the heat liberated, but also the threshold current that can be passed through the circuit. If a current above a certain value is passed, it would fuse the bulb. A device called an ammeter is going to help Sarvesh detect the amount of current flowing in the circuit. In this article, we will learn about ammeters in detail.

Table of contents

• What is an ammeter?
• What is a galvanometer?
• Conversion of galvanometer into ammeter
• Practice problems
• FAQs

What is an ammeter?

Ammeter is a device used for measuring currents. On the other hand, a galvanometer is used to measure smaller currents. An ammeter is always connected in series in a circuit. It has a needle which deflects on a graduated scale. The scale is graduated in amperes () or milliamperes( An ideal ammeter is one which has zero resistance.

Let us consider the following circuit where indicates the resistance of the ammeter and indicates the external resistance. The current flowing in the circuit can be written as, 

.

  by definition of an ideal ammeter. 

I.e. the reading would be the same as when a resistor is connected. From equation it is clear that the lower the resistance of the ammeter, the more accurate the reading will be.

What is a galvanometer?

A galvanometer is a device used for detecting and measuring small currents. A special type of galvanometer is the moving coil galvanometer. It consists of a rectangular coil with many turns made of insulated copper wires having a thin cross section. The coil is suspended between two horseshoe magnets by means of a fine phosphor bronze strip. The horse shoe magnets provide a radial magnetic field to ensure that the plane of the coil is parallel to the magnetic field at all times. In other words, the normal drawn to the coil is perpendicular to the magnetic field provided by the horseshoe magnets.

Conversion of galvanometer into 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. Then, will be 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 .

Practice problems

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

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

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

The value of shunt resistance that needs to be connected is  

Solving, we get

Q. A toaster equipment having a resistance of is connected to an external resistor of The emf of the battery supplying current is An ammeter is connected in series with both the toaster and the external resistor. Calculate the current flowing in the circuit?

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

A. b

Emf

Total resistance of the circuit,

Current flowing in circuit

Q. Calculate the shunt that has 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)

A. d

Let indicate the main current and indicate the galvanometer current. 

Given

Q. A battery having emf of is connected in series with a resistor of An ammeter having resistance is connected in series in the circuit. Calculate the ammeter reading?

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

A. a

Given ,

Current flowing in the circuit,

FAQs

Q. What is the difference between an ammeter and a 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 so that the actual current reading of the branch in which current is to be measured does not change. 

Q. What is the D'Arsonval movement?
A. The current passing through the coil of an ammeter causes the needle attached with it to undergo deflection. The mechanism that governs this deflection is called the D'Arsonval movement. 

Q. Write the expression for the effective resistance of the circuit 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