Page 838 - Basic Electrical Engineering

In dynamometer instruments, torque is developed due to the interaction of
the two magnetic fields produced by the current flowing through the fixed
coils and the moving coil.

The control torque is provided by a set of helical springs attached to the
spindle such that, T = Kθ. where K is the spring constant.
c

Figure 11.22 Connection diagram of a dynamometer-type ammeter or voltmeter

The deflecting torque T is proportional to the product of the current
d
flowing through the fixed coils and the moving coil. As in ammeter and

voltmeter, same current flows through these coils; therefore

T α I 2
d

and

T α θ
c

Thus,

θ α I 2

The instrument will follow a square law in its deflection, and hence the

scale will be non-uniform one.
The torque developed will always be positive irrespective of the direction
of current. Therefore, the instrument can be used for both ac and dc

measurements.