What is Vibration Galvanometer : Types, Construction and Theory

The galvanometer is an instrument which is used to measure or detect the small amount of current. It is an indicating instrument and it is also a null detection that indicates a null detector, such that no current is flowing through the galvanometer. The galvanometers are used in bridges to show the null detection and in potentiometer to show the small amount of current, The AC galvanometers are of two types they are phase sensitive galvanometer and frequency sensitive galvanometer. The vibration galvanometer is one type of frequency sensitive galvanometer. This article discusses the vibration galvanometer.

What is Vibration Galvanometer?

The galvanometer in which the measured current and the oscillation frequency of the moving element becomes equal is called the vibration galvanometer. It is used to measure or detect a small amount of current.

Difference Between the Types of Vibration Galvanometer

There are two types of vibration galvanometers they are moving coil type vibration galvanometer and moving magnet type vibration galvanometer. The difference between moving coil type vibration galvanometer and moving magnet type vibration galvanometer is shown in the below table.

Construction

The construction of the vibration galvanometer has permanent magnets, a bridge piece that is used for the vibration, mirror which reflects the beam of light on the scale, pulley which tightens the spring and the vibration loop.

As the basic principle of the galvanometer is, when a current source is applied across the coil then the electromagnetic field is produced in the coil which moves the coil. The same principle is applicable to the above figure. When the coil is moving then it creates vibration in the vibrator loop and the beam of light is passed on the mirror which reflects the vibration and the beam of light with respect to the vibration on the scale and the spring is used for the controlling of the vibrator loop. The frequency range is used to measure is 5 Hz to 1000 Hz, but we basically use 300 Hz for the stable operation and it has good sensitivity at 50 Hz frequency.

Theory

Let the value of current passing through the moving coil at an instant t be

I=I_m sin(ωt)

The deflecting torque produced by the galvanometer is expressed by

T_d=Gi= I_m sin(ωt)

Where G is the galvanometer constant
The equation of motion is expressed as

T_J+T_D+T_C=T_d

Where T_J is the torque due to moment of inertia, T_D is the torque due to damping, T_C is the torque due to spring, and T_d is the deflecting torque.

J d²ϴ/dt²+ D d²ϴ/dt²+ Kϴ=GZ sin(ωt)

Where J is the inertia constant, D is the damping constant, and C is the controlling constant.
After the solution of the above equation will get the deflection (ϴ) is

ϴ=G GI_m/√(Dω)²+(K-Jω²)² * sin(ωt- α)

The amplitude of vibration is expressed as

A=GI_m/√(Dω)²+(K-Jω²)²

The vibration galvanometer amplitude is increased by increasing the galvanometer constant (G). To make the amplitude large by increasing either galvanometer constant (G) or decreasing

Case 1 – Increasing Galvanometer Constant (G): We know that the galvanometer constant is given by

G = NBA

Where N is the number of turns of the coil, B is the flux density, and A is the area of the coil.
If we increase the number of turns (N) and area of the coil (A) then the galvanometer constant increases, but the moment of inertia is also increased due to the heavy mass of the coil. So √(Dω)²+(K-Jω²)² will increase.

Case 2 – Decreasing √(Dω)²+(K-Jω²)²: Where J and D are fixed, K can be changed by adjusting the length of the spring. So √(Dω)²+(K-Jω²)²  should be minimum. 

For the minimum value we can put (K-Jω²)² =0 

or  ω=√K/J⇒2ᴨf=√K/J

Supply frequency f_S=1/2ᴨ * √K/J

For maximum amplitude, the natural frequency should be equal to supply frequency f_s=f_n 

So that the amplitude of the vibration should be maximum. Thus, the vibration galvanometer is tuned by changing the length and tension of the moving system in order that the natural frequency of the moving system is equal to the supply frequency. So that the stable operation of the vibration galvanometer is achieved.

Thus, this is all about an overview of vibration galvanometer, construction of vibration galvanometer, theory, and the difference between the types of vibration galvanometer are discussed. Here is a question for you, what is the advantage of a vibration galvanometer?