Galvanometer

You might have seen a device with a G symbol in your physics lab a lot many times. You may have thought what this device is and what is the use of it?

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Well, this device is a galvanometer. Now,

What is a Galvanometer?

A galvanometer is a device that measures or detects small currents with appropriate modification. It can be converted into ammeter to measure the currents in the order of an ampere or millimetre or in the range of milliamperes or microammeter to measure microampere current.

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So, the use of a galvanometer is to detect whether there is a current in the circuit or not. While measuring the current, we have to convert it into an ammeter.

There are many types of galvanometer. In this article, we will study the moving coil type galvanometer.

Principle of Moving Coil Galvanometer

The principle of moving coil galvanometer is a torque on a current loop placed in a magnetic field.

Function of Galvanometer

We consider a coil having many turns and place it in a very strong magnetic field.

The reason is the more the number of turns, the more is current and more is the torque produced. Similarly, stronger the magnetic field; more is the torque produced.

The small current that needs to be detected/measured is sent to the coil. A torque acts on the coil which rotates the coil.

Now, this coil keeps on rotating. How to count the number of deflections it makes?

Here, we use a pointer and a scale to get the deflection of the coil.

So, more the current, more is the torque, more is the deflection (rotation), more is the reading on the scale.

The purpose of this is to make the current directly proportional to the deflection.

Construction of Moving Coil Galvanometer

  1. Take two terminals, T1 and T2.

  2. Hang a fine wire through a metal attached to terminal T1.

  3. Tie a coil to the free end of the wire

  4. Attach one end of a spring to the bottom of the coil and another end to the terminal T2.

  5. The two horseshoe electromagnets are placed around this coil.

  6. Make sure the coil is tilted because torque isn’t generated when the coil is parallel to the magnetic field.

Now, let’s look at the moving coil galvanometer diagram:

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  1. The N turns wire on the coil should be insulated.

  2. The spring is made of either phosphor bronze or quartz wire.

  3. We attach a concave mirror to the wire at the top of the coil so that deflection can be measured using a lamp and scale.

Initially, this circuit is open. On joining both the terminals, the current I starts flowing. 

Let the area of each turn be A, and the magnetic field be B.

As the circuit completes, i.e., on making the connection between the wire, torque starts generating.

As the current (that is to be measured) is sent to the coil. We can express the torque produced as:

                               て = NIAB Sin Ө 

Here, Ө is the angle between the area vector and the magnetic field. This area vector A is perpendicular to the plane of the loop.

Working of Moving Coil Galvanometer

As the coil rotates, it rotates smoothly and the spring twists. Therefore, a restoring torque develops on the wire and the spring.

Slowly as the coil rotates, the restoring torque in the spring starts obstructing in its rotation. 

This means more we rotate the coil, more is the restoring torque in the wire and the spring.  

So, more is the twist; more is the restoring force. This strain in the wire is the Torsional strain. Now, how to measure this Torsional strain?

For example, if we rotate the coil by 1°, the Torsional constant will be C, similarly, if we rotate by 2°, it is 2 C. However, N, I, and A remain constant.

The restoring force in the spring and the wire keeps on increasing. However, the torque on the coil remains the same. A time comes when this applied torque balances the restoring torque.

Therefore, on rotation by  Φ°, it will be CΦ. So, the formula for the restoring force is:

                 てRestoring torque = CΦ

 If CΦ is the twist, restoring torque is CΦ, where C is the restoring force caused by a unit degree rotation. 

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Initially, this pointer points to 0. As the torque rotates, the deflection occurs as shown below:

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The pointer fixes at the point, and we get the final reading on the scale.

At this time, てapplied  = てRestoring torque                  

         NIAB Sin Ө = CΦ 

         I =  \[\frac{C}{N ABSin \theta}\]Φ

Here, Φ and Sin Ө is a variable, and I α Φ/ Sin Ө.

But, we desired I α Φ. Now, to remove this Sin Ө, we use the radial field.

Now, we will use a soft iron core (it is a strong ferromagnetic material) in place of a loop and cylindrical magnets in place of horseshoe magnets. This is how we can create a radial field. A field in which the magnetic field lines pass from N to South pole such that the area vector Ais always perpendicular (radial) to the magnetic field B.

We use the radial field because it increases the strength of the magnetic field around the coil.

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This means Ө between B and Awill always be 90°, i.e., Sin Ө becomes 1. Therefore, our purpose is solved, i.e., 

   I =  \[\frac{C}{N ABSin \theta}\]Φ  = \[\frac{C}{N AB}\]Φ  or I α Φ

FAQ (Frequently Asked Questions)

Q1: Write the Types of Galvanometer.

Ans: The types of galvanometer are:

  1. Tangent galvanometer

  2. Static galvanometer

  3. Mirror galvanometer

  4. Ballistic galvanometer

Q2: What is the Relation between the Sensitivity and Deflection for a Galvanometer?

Ans: The sensitive galvanometer shows a huge deflection in a small current. A moving coil galvanometer is a highly sensitive instrument. Because of this nature, it can detect current in the range of milliamperes. 

Q3: What are the Current Sensitivity and Voltage Sensitivity of a Galvanometer?

Ans: The current sensitivity of a moving coil galvanometer is given as:

                                    CSenstivity = Φ/I = NAB/C

This means in the fractional flow of current, there is a high deflection.

If galvanometer shows a high deflection in a small voltage, then it is a voltage sensitivity given by

                                  VSenstivity =  Φ/V = NAB/CR 

Here, V = I/R (R = The resistance of the coil).

This is how we can convert galvanometer to ammeter and voltmeter to get the value of current sensitivity and voltage sensitivity respectively.

Q4: How Do You Increase the Sensitivity of a Galvanometer?

Ans: You can increase the sensitivity by:

  1.  Increasing number of turns

  2.  Increasing area of coils

  3. Magnetic field (using radial field)

  4. Decrease C by using phosphor bronze or Quartz wire because their Torsional constant is very low