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Galvanometer and ammeter

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Galvanometer and ammeter are both current measuring devices.The galvanometer is used to measure very small current while ammeter is used to measure very large current up to 10 A.

Now!

We study in detail about these two instruments and we will also see the difference between galvanometer and ammeter.

What is galvanometer?

It is an electrical device which is used to detect the passage of current through a circuit.

Construction:

This device consists of following parts:

  1. A U shaped magnet with concave shaped north and south poles.
  2. A rectangular coil C made up of enameled copper wire suspended between poles of the U-shaped magnet.
  3. A non-magnetic frame over which the wire is wound.
  4. A suspension wire F acting as one current lead.
  5. A loosely wound spiral E  acting as second current lead.
  6. An iron cylinder D placed inside the coil to concentrate the field through it.

How does it work?

When the current is passed through the coil, a couple of two equal and opposite forces acting on two different points act on the coil and the coil tends to rotate.Such couple is known as deflecting couple.

If N is the number of turns of the coil and A is the area then the torque due to the deflecting couple is given by:

τ = NIBA cosα

Since the pole pieces of the magnet are made concave to make the field radial, therefore the plane of the coil is always parallel to the field, so α =0º

τ = NIBA cos0º

τ = NIBA(1)

τ = NIBA …………………….(1)

Restoring torque

As the coil turns under the action of deflecting torque, the suspension wire is twisted which gives rise to the torsional couple which tends to untwist the suspension wire and restores the original position.This couple is known as restoring couple.

The torque due to restoring couple is proportional to the angle of deflection θ as long as the suspension wire obeys’s Hook’s law.Thus

Restoring torque ∝ θ

Restoring torque = Cθ —————(2)

Where C is the constant, known as twisting or torsional constant and it depends upon the nature of suspension wire.

When the deflecting couple balances the restoring couple then the coil will come to rest.So, in equilibrium.

Deflecting torque =Restoring torque

NIBA = Cθ

oR

I = Cθ/NBA

Since C/NBA =constant then:

I ∝ θ

So the current passing through the coil is directly proportional to deflection θ.

How can we measure the angle of deflection of moving coil galvanometer?

There are two methods to determine the angle of deflection of the coil.

1: Lamp and Scale arrangement

In a sensitive galvanometer, the angle of deflection is observed by means of the small mirror attached to the coil along with the lamp.

A beam of light from the lamp falls on the mirror of the galvanometer.After reflection from the mirror, it produces a spot on a transparent scale placed at a distance of one-meter form galvanometer.When the coil rotates, the mirror attached to the coil also rotates and the spot of light moves along the scale.

The displacement of the spot of light on the scale is proportional to the angle of deflection, provided the angle of deflection is small.

2: Pivoted galvanometer

In this method, the coil is pivoted between two jeweled bearings.The restoring torque is provided by two hairsprings which also act as current leads.An aluminum pointer is attached to the coil which moves over a scale.

Define current sensitivity of the galvanometer.How can it be increased?

The angular twist per unit is called current sensitivity of a galvanometer.It is expressed as:

Sensitivity = θ/I = BAN/C

We can make it sensitive by following steps:

  • By increasing the number of turns of the coil N, but if it increased beyond a certain limit then the coil become heavy.
  • By increasing magnetic flux density B, placing the soft iron cylinder inside the coil.This change is most effective to increase the sensitivity.
  • By increasing the area A but it can make the coil bulky and unmanageable.
  • By decreasing  C ,a suspension wire should have large length and small radius.

What is  Deadbeat galvanometer?

A type of galvanometer,in which the coil comes to rest quickly after passing of current through it,is called stable or deadbeat galvanometer.

How can we make Deadbeat galvanometer?

A sensitive coil is not stable; its coil oscillates many times before coming to rest, although the current through the coil is stopped.But in some galvanometers, the coil comes to rest very soon, as the current stops the flow through it.

When current passes through the coil is starts rotating about the central axis.The flux through the frame of coil changes so (by Faraday’s law) an emf and hence a (eddy) current is induced in the frame which opposes it cause (i.e. the rotation of coil).In this way, the oscillations are damped.

Ammeter

An electrical instrument which is used to measure current passing through it circuit is called ammeter.

Principle and Construction

Its principle and construction is similar to the moving coil galvanometer.

 Galvanometer to Ammeter

To current a galvanometer into an ammeter a very small resistance is connected in the parallel with the galvanometer. This resistance is called shunt resistance (Rs).

Watch video about the conversion!

Related topics:

  • Difference between ammeter and voltmeter
  • Potentiometer
  • Wheatstone bridg

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