Kelvin’s Current Balance

Principle. Two conductors carrying current in the same direction experience a force of attraction. If the conductors carry current in opposite directions, they experience a force of repulsion. 

Construction and Working. This current balance used four fixed coils A, B, C, D and two movable coils E, F all joined in series. The coils E and F are kept on a lever PQ capable of rotation about the axis XY. The current in the coils A, B and F is clock wise and that in the coils, C, D and E is anti-clockwise when viewed from the top.

Let i be the current passing through the coils. B and F are attracted (same direction) while D and F are repelled (opposite direction). Similarly C and E are attracted, while A and E are repelled. Consequently, the beam gets titled. To bring the beam back to the horizontal position, the rider of weight W is moved to the right by a distance d.

Then, for equilibrium
        Wd = K i²
Here, K is a constant dependent on the dimension of the instrument.
. :            i = √Wd/K

The beam is initially calibrated by passing known currents.
For measuring different ranges of current, different weight are used.
The instrument can be used to measure direct as well as alternating current as i² ∞ d.



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