The force between moving charges

The classic theory of circuits, as used by electrical engineers, is founded on the assumption that charge is invariant. The new forces which come into play when charges move, that is, when current flows, are expressed in terms of magnetic fields. According to this view the force between a pair of relatively moving charges is made up of two components, one along the line joining them which is always given by Coulomb's law, and a second perpendicular to this, depending upon the velocity, and expressed by means of magnetic fields. This classic concept involves necessarily a change of mass with velocity in order to account for observed results with high-speed electrons. Such a change of mass is also given by the theory of relativity.