A moving-coil galvanometer is based on the principle that a current-carrying conductor in a magnetic field experiences a force. It is used to measure or detect small currents. It is made up of:
• A rectangular coil of wire wrapped around a cylindrical laminated soft iron core. Current flowing through the coil produces a magnetic field around it.
• A magnet between whose poles the coil is free to rotate on an axis.The magnet creates the torque on the coil by interacting with the magnetic field around it.
• A spring which is connected to the axis on which the coil rotates.The spring produces a couple in the opposite direction to one produced by the magnet.
• A pointer which is connected to the axis. The angle through which the pointer rotates is proportional to the current flowing through the coil.
• A scale from which the current flowing through the coil can be read, as shown in Fig 24.7.This is the function of the galvanometer.
Pointer N S
Uniform scale Spring
Moving coil Fixed soft-iron core
Factors affecting the effectiveness of the moving-coil galvanometer
• The coil of wire is wound around an aluminium frame to ‘damp’ the movement of the coil and bring the point to rest quicker.
Fig 24.7: Moving-coil galvanometer Current
• The galvanometer has curved pole faces in order to give a radial magnetic field.This produces a constant torque on the coil, which in turn allows for a uniform scale.
Conversion of a Galvanometer Low resistance Fig 24.8: Path taken by the current in a galvanometer
SAMPLE PROBLEM 24A Æ
to an Ammeter A moving coil galvanometer can be converted to an ammeter by connecting a small resistance in parallel with it.This resistance is called a shunt.
A galvanometer has a resistance of 10 and gives a full-scale deflection with a current of 5 mA. (i) Find the resistance of the shunt (resistor) that must be connected in parallel to enable the galvanometer to measure currents of up to 1 A.
(i) The voltage across the galvanometer and the resistor is the same.
The maximum current flowing through the galvanometer is 5 mA; this occurs when the current in the circuit is 1 A; which implies the current flowing through the resistor is 995 mA.
The resistance of the shunt is 0.0503 .Æ (995)Rshunt (5)(10)QRshunt 0.0503
