The revolving conducting rings demonstration of magnetic forces consisted of two circular windings with the outer one fixed and the inner one able to rotate freely, but biased to rest in a plane perpendicular to the first. The windings were connected in series. When a current was passed through the windings, the planes of the two rings became parallel. This is easily understood on the basis of the attraction of parallel currents. If this current is large enough, the effects of the terrestrial magnetic field can be neglected.
The apparatus is represented schematically in the figure to the right, with the two rings happening to be at right angles to each other. The magnetic field is in the direction shown by the B vectors on both sides of the fixed coil. Since the current is in opposite senses on the two sides, the forces F are also opposite, and tend to rotate the inner ring in the direction that brings parallel currents together.
If a compass needle is placed at the centre of the fixed winding, it will point in the direction of the resultant of the field due to the winding and the horizontal component of the terrestrial field. If the current is sufficiently large, this direction will be perpendicular to the plane of the winding. This is the principle of the tangent galvanometer, which compares the field of the winding to the horizontal terrestrial field of magnitude roughly 0.3 gauss. From the field, the current in the winding can be found. This demonstration clearly shows that the magnetic moment of a coil is perpendicular to the plane of the coil, and the moments of the two coils tend to align.
The apparatus is illustrated, and erroneously described, in Early Apparatus.
Composed by J. B. Calvert
Created 24 November 2010