Principle construction and working of moving …

MOVING COIL GALVANOMETER:

• Moving coil galvanometer is an electromagnetic device that can measure small values of current. It is also known as Weston galvanometer.
• It works on the principle that when a current loop is placed in an external magnetic field, it experiences torque, and the value of torque can be changed by changing the current in the loop
• Moving coil galvanometer consists of permanent horse-shoe magnets, coil, soft iron core, pivoted spring, non-metallic frame, scale and pointer
• We know that a current loop having N number of turns,and the cross sectional area A, carrying current i, when placed in and along the direction of external magnetic field B, experiences a torque given by:

ԏ = NiAB

The pivoted spring of spring constant k would oppose the above torque with restoring torque C given by: C = kΦ

Here, Φ is the angular deflection of spring

• Both, the torque, and the restoring torque would be equal:

kΦ = NiAB

Φ = NiAB/k

• In the above equation, except for current, every other quantity on the right hand side is constant for a galvanometer, hence:

Φ ∝ i

• So, the angular deflection Φ produced in the pointer could be measured in terms of current in the scale calibrated on the basis of above equations.
• To use galvanometer as an ammeter (to measure higher values of current), we need to connect a shunt wire, with very small resistance(Rs), in parallel with the galvanometer (which have very low resistance of Rg

Equivalent resistance R of ammeter will be:

R = RgRs/(Rg + Rs)

Rg ˃˃ Rs

∴ R = RgRs/Rg = Rs

• So, the equivalent resistance of ammeter is very less, which is a must for sensitivity of ammeter to be higher. Also, most of the current will pass through the shunt, thus protecting the galvanometer from any damage.
• Ammeter is connected in series with the circuit where current is to be measured
• Current sensitivity(deflection per unit current) of galvanometer is given by:

Φ/i = NAB/k

• To use galvanometer as a voltmeter, we need to connect a wire, with very high resistance(Rw˃˃Rg), in series with the galvanometer to ensure that our voltmeter equivalent resistance is high and so that it will draw a very small current. Equivalent resistance will be given by:

R = Rg + Rw = Rw

Voltmeter is connected in parallel with the circuit where voltage is to be measured

• Voltage sensitivity (deflection per unit voltage) of galvanometer is given by:

Φ/V = NAB/(kR)