These instruments are widely used in laboratories and switch board at commercial frequencies because these are cheaper in cost, robust in construction and can be manufactured with required accuracy. These are generally of two types:- 1. The attraction type.2. The repulsion type.
The attraction type instrument operate on the principle of attraction of a single piece of soft iron into a magnetic field and repulsion type instrument operate on the principle of repulsion of two adjacent iron pieces magnified by the same magnetic field. Repulsion type instrument are more sensitive than attraction type instrument as in repulsion type instrument large separating torque is developed by having two iron element positional class together inside the field coil where the magnetizing effect is maximum.
In both type of these instruments, the current under measurement is passed through a coil of wire. This current carrying coil set up the necessary field depending on the magnitude of the current to be measured. The coil may be of a few turns of very heavy conductor or of many turns of fine wire. The instrument to be used as an ammeter is provided with a coil of few turns of thick wire in order to have low resistance and carry large current and that to be used as a voltameter is provided with a coil of large number of turns of wire in order to have high resistance and draw as small current as possible.
EXPRESSION FOR TORQUE
Let L be the self inductance corresponding to a total angular deflection of q radians and change in inductance be dL correponding to small change in deflection angel dq due to small change in current.
The change in energy of magnetic field,
dw = Td dθ
Since change in energy dE = workdone, dw
Td dθ = ½ I2dL
Td = ½ I2dL/dθ
where I is in amperes, L is in Henry and θ is in Radians.
Thus toruqe is proportional to the square of the instrument current and to the rate of change of inductance with deflection.