| Almost every physics major does an experiment
with a current balance. The students measure the force of repulsion between
two straight, parallel wires, one fixed, and one moving, by measuring the
gravitational force which just balances the magnetic force. The force is
proportional to the product of the currents in the two conductors, and depends
on the permeability of free space.
Lord Kelvin’s original current balance design dates from 1882. Instead of parallel straight wires, the current being measured passes through parallel circular loops of wire. A pivoted beam carries a horizontal coil on either end. Above and below this coil are fixed coils, and the same current passes through all of them. Connections are made to the coils to cause an attraction of the left-hand pivoted coil to the coil below, and a repulsion from the coil above, and the beam moves down on the left-hand side. On the right-hand side, connections are made to cause this end of the beam to move up. The counter-clockwise torque produced by the magnetic interaction is proportional to the square of the current through the coils.
To counteract this torque, a cylindrical weight sliding in a V-groove in the beam running across the front of the balance is moved to the right, producing a clockwise torque. The balance is adjusted until that the pointer on the right-hand end of the scale returns to the zero position, and the distance the mass has been moved is proportional to the square of the current, resulting in non-linear markings on the scale. The scale extends the entire length of the beam; the system is in balance when the sliding weight is at the left-hand end of its excursion. Various current scales are obtained by using different sliding weights, which are shown in the picture of the Washington and Lee apparatus.
| The very heavy, braided conductors leading from the left
side of this Kelvin current balance show that it was adapted to measure large
This example is in the Garland Collection of Classical Physics Apparatus at Vanderbilt University, and probably dates from the late 1870s.