Ampere Current Balance

Principle. This instrument makes use of the force between two current carrying conductors to measure an unknown current in terms of mass, length and time only.

Construction and Working. It is made up of a carefully wound coil C kept between two fixed coils A and B. The same current is passed through the three coils but the senses are so adjusted that the sense of current in B is opposite to the common sense of current flow in A and C. both A and B then exert a downward force on B of magnitude
            F = ki2                                     … (1)

Here k is a constant dependent on the geometry of the coils.
The force F is counter balanced by the weight of suitable mass M and a balance arranged as shown is used to check this. It follows then that
            F = ki2 = Mg
. :            i = √Mg/k