Moving Coil Galvanometer

A moving coil galvanometer is a device which is used to detect and measure small electric current. In fact, it is the basic components of most of the measuring instrument including ammeters and voltmeters. It is the most sensitive instrument.

Principle: it operation is based on the principle that when a current carrying loop or coil is placed in a uniform magnetic field, the coil experiences a torque.

Construction: it consists of a rectangular coil having large number of turns wound on a non-metallic frame. The coil is suspended between two poles of a permanent magnet which are cylindrical in shape. The coil is suspended by a phosphor – bronze strip which acts as path for the current to the coil. The strip is finally connected to the terminal T2 of the galvanometer. The other end of the coil is connected to a light spring which is finally connected to the terminal T1 as shown in figure 1. The spring exerts a very small restoring couple on the coil. A piece of soft iron is placed within the frame of the coil. A plane circular mirror M is attached to the suspension to note the deflection of the coil using lamp and scale arrangement.

diagram-of-moving-coil-galvanometer

Theory:

Let,

B = magnetic field intensity

I = current passed through the coil

l = length of the coil

b = breadth of the coil

l x b = A = area of the coil

N = number of turns of the coil

When current flows through the coil, it experiences a torque, which is given by

τN = BINA sin θ

where θ is the angle made by the normal to the plane of the coil with the direction of the magnetic field. If this angle is 900, then sin θ = sin 900 = 1. [it is possible when cylindrical poles of permanent magnet are used which produce radial magnetic field.]

Then,

τN = BINA

This torque is known as deflecting torque.

As the coil gets deflected, the suspension wire is twisted and a restoring torque is developed in it. If k is the restoring torque per unit twist of the suspension wire, then the restoring torque for the deflection α is given by

τ’N = kα

For equilibrium of the coil, deflecting torque = restoring torque

i.e. BINA = kα

or           I = \dfrac{k \alpha}{BNA}

or         I = G α

where  G = \dfrac{k}{BNA} is called galvanometer constant.

Thus, deflection of the coil is directly proportional to the current flowing through it. Hence, we use a linear scale in the galvanometer to detect the current flowing in the current.

Sensitivity of a Galvanometer

A galvanometer is said to be sensitivity if small amount of current flowing though the coil of galvanometer produces large deflection in it. A galvanometer can be converted into ammeter or voltmeter so it has two types of sensitivity.

  1. Current Sensitivity

The current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit current flowing through it.

i.e.  current sensitivity = \dfrac{\alpha}{I} = \dfrac{\alpha BNA}{k\alpha} = \dfrac{BNA}{k}

Current sensitivity of a galvanometer can be increased either by

  1. Increasing the magnetic field B by using strong permanent horse shoe shaped magnet.
  2. Increasing the number of turns N. But number of turns of the coil cannot be increased beyond a certain limit. This is because the resistance of the galvanometer will increase subsequently and hence the galvanometer becomes less sensitivity.
  3. Increasing the area of the coil A. But it will make the galvanometer bulky and ultimately less sensitive.
  4. Decreasing the value of restoring force constant k by using a flat strip of phosphor – bronze instead of circular wire of phosphor – bronze. Quartz fibers can also be used for suspension of the coil because they have large tensile strength and very low value of K.

2. Voltage Sensitivity

Voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit voltage applied to it.

i.e.  voltage sensitivity = \dfrac{\alpha}{v} = \dfrac{\alpha}{IR} = \dfrac{BNA}{kR}

where  \alpha = \dfrac{BINA}{k}

Voltage sensitivity can be increased by

  1. Increasing number of turns of the coil (N)
  2. Increasing magnetic field intensity (B)
  3. Increasing area of the coil (A)
  4. Decreasing restoring torque per unit twist of the suspension (k)
  5. Decreasing resistance (R)

Advantage of Moving Coil Galvanometer

  1. The sensitivity of the galvanometer can be increased by increasing N, B and A while decreasing the value of k.
  2. The instrument has a linear scale.
  3. Since the instrument uses high value of B, the deflection is undisturbed by the earth’s magnetic field.
  4. As the coil is wound on a nonmagnetic metallic frame, damping is produced by eddy currents. As a result the coil quickly assumes the final position.