Mechanical force between electric circuits

J. Slepian: The relation which is perhaps most frequently used by electrical engineers for calculating mechanical forces between circuits is that due to Maxwell, which states that, in any displacement in which currents are kept constant, the mechanical work received during´ the displacement is equal to the increase in magnetic energy of the system. This relation is readily derived from the principle of conservation of energy by taking into account the electrical input during the displacement. As Professor Karapetoff puts it, there is a “fifty-fifty” rule here; half the electrical input goes to increasing the magnetic energy and half is given up to mechanical work.