Definition: The galvanometer is the device used for detecting the presence of small current and voltage or for measuring their magnitude. The galvanometer is mainly used in the bridges and potentiometer where they indicate the null deflection or zero current.
Principle of Galvanometer
The potentiometer is based on the principle that the current sustaining coil is kept between the magnetic field experiences a torque.
Construction of the Galvanometer
The construction of the potentiometer is shown in the figure below.
Moving Coil – The moving coil is the current carrying part of the galvanometer. It is rectangular or circular in shape and has the number of turns of fine copper wire. The coil is freely moved about its vertical axis of symmetry between the poles of a permanent magnet. The iron core provides the low reluctances flux path and hence provides the strong magnetic field for the coil to move in.
Suspension – The coil is suspended by a flat ribbon which carries the current to the coil. The other current carrying coil is the lower suspension whose torque effect is negligible. The upper suspension coil is made up of gold or copper wire which is made in the form of a ribbon. The mechanical strength of the wire is not very strong, and hence the galvanometers handle carefully without any jerks.
Mirror – The suspension carries a small mirror which casts the beam of light. The beam of light placed on the scale on which the deflection is measured.
Torsion Head – The torsion head is used for controlling the position of the coil and for adjusting the zero setting.
Working of Galvanometer
Let, l, d – the length of respective vertical and horizontal side of the coil in the meter.
N – number of turns in the coil,
B – Flux density in the air gap, wb/m2
i – current through moving coil in Ampere
K – spring constant of suspension, Nm/rad
θf – final steady state deflection of moving coil in radiance
When the current flows through the coil, it experiences a torque which is expressed as
The above equation shows that when the mirror turns through an angle θF the reflected beam turns through an angle 2θF shown in the figure below.
The galvanometer is used as an ammeter by connecting the low resistance wire in parallel with the galvanometer. The potential difference between the voltage and the shunt resistance are equal.
Where, S = shunt resistance and Is = current across the shunt.
The value of the shunt current is very small as compared to the supply current.
Conversion of Galvanometer into a voltmeter
The galvanometer is used as a voltmeter by connecting the high resistance in series with the circuit.
The range of the voltmeter depends on the value of the resistance connected in series with the circuit.