Page 835 - Basic Electrical Engineering

strong magnetic field that exists inside the coil irrespective of the direction of
current flow through the coil. The deflection of the pointer has to be
controlled. The control torque is provided by a spring. The control torque is

proportional to deflection θ. The deflecting torque is proportional to the
square of the current.

T α I 2
d
T α θ
c

where θ is the angle of deflection of the pointer over the scale.

T is the deflecting torque.
d
T is the control torque.
c

For a steady deflection, T = T c
d
Therefore, θ α I 2

Figure 11.20 (a) Attraction-type moving iron instrument; (b) air friction damping mechanism

Deflection of the pointer is proportional to the square of the current
flowing through the coil. The scale of the instrument is therefore a cramped

scale, i.e., there will be congestion at the beginning of the scale. The
instrument will measure both ac and dc and deflection of the pointer will be
unidirectional. Damping, i.e., reduction of oscillation of the pointer is

achieved by air friction damping mechanism where the base of the pointer is
attached to a very light piston placed inside an air chamber as has been

shown in Fig. 11.20 (b) where a light aluminium piston attached to the