galvanometer

A MOVING COIL GALVANOMETER

It is a device used to detect and measure small current in a circuit.
Principle: A current carrying coil placed in a magnetic field experiences a torque.
Construction: It consists of a narrow rectangular coil having a large no. of turns of copper wound on a metallic frame. It suspended between a semicircular poles pieces of power horseshoe magnet by mean of a suspension wire made of phosphorus bronze with a plane mirror attached to it. A soft iron core is fixed within the coil. This marks the magnetic field linked with coil to be radial. At the lower end coil is attached to affine spring. Lamp and scale arrangement is used to measure deflection.

Construction

Working: When the current I is passed through the coil, experience a torque given by
Where N = no. of turns
A = area of coil
B = magnetic field
θ=angle between normal coil and magnetic field
Since magnetic field is radial, so Ɵ = 90° therefore
Since magnetic field is radial, so Ɵ = 90° therefore
τ=NIAB
Due to this torque, coil starts rotating. So the suspension wire gets twisted and a restoring torque is developed in the suspension wire made of phosphor and bronze. The restoring torque continues to increase and stages reach when the restoring torque is equal to the deflecting torque.
At equilibrium    Deflecting torque = Restoring torque
NIAB = kα
Where α is the angle by which the coil rotated and k is the restoring torque per unit angle.
Therefore                      I = kα/NBA = Gα
Where G = k/NBA is called galvanometer constant.
I∝α
Hence current is directly proportional to the angle by which the coil rotated. Also light from lamp falls on the mirror gets deflected and falls on the scale. When the current passes the mirror moves and light spot moves on the scale.
Sensitivity: A galvanometer is said to be sensitive, if it give large deflection, even when a small current is passed through it or a small voltage is applied across its coil.
Current sensitivity: It is defined as the deflection produced in the galvanometer on passing current of one ampere. i.e.

Current sensitivity

Voltage sensitivity: It is defined as the deflection produced in the galvanometer where a potential difference of one volt is applied its coil.
Voltage sensitivity
Note: A galvanometer is said to be highly sensitivity, if
  1. N is large
  2. B is large
  3. A is large
  4. k is small
  5. R is small
However, N and A cannot be increased beyond a certain limit as otherwise the size and resistance of galvanometer will become large. Therefore B is made as large as possible and k is as small as possible. To decrease k the suspension wire is made of phosphor bronze. Restoring torque per unit twist may further be decrease by taking the suspension wire in the form of strip.
Comment Below, If any problem occurs.
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By Pooja Srivastava

My name is Pooja Srivastava, if you have any query you can contact me at pujasrivastava1995@gmail.com

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