Page 836 - Basic Electrical Engineering

spindle moves in a fixed air chamber which is closed at one end. Due to
pressure difference created by the movement of the piston, a damping torque
is developed which reduces any oscillation of the pointer on the scale when

measurement is taken.

Repulsion-type MI instruments: Like in attraction type, the fixed coil carries

the current or current proportional to the voltage to be measured. On the inner
surface of the coil, a specially shaped iron piece is attached. See Fig. 11.21
(b). This is the fixed iron piece. On the spindle is attached another piece of

iron in the form of a fin. This is the moving iron. The current passing through
the coil produces a magnetic field. In this magnetic field are placed both the

fixed iron and the moving iron. Both these iron pieces will be similarly
magnetized due to the influence of the magnetic field. This will create a force

of repulsion among the two pieces of iron. One piece of iron being fixed, the
other piece of iron will move which will create a deflecting torque. This

deflecting torque has to be opposed by a control torque such that under steady
deflection, T = T .
c
d

Figure 11.21 (a) Repulsion-type moving iron instrument; (b) another view of a repulsion-type MI
instrument

If T is more than T , the pointer will move. Its movement will stop when
c
d
the deflecting torque, T equals the control torque, T . Since the magnitude of
c
d
the deflecting torque depends upon the magnetism of the two pieces of iron,