Current strength. When two conductors at different potentials are connected by a wire there is a gradient of potential along the wire and equilibrium no longer exists, but there is a transfer of electricity called a current in the wire from one conductor to the other. The strength of the current in the wire may be measured by the rate of increase of the positive charge on one of the conductors, say C = dQ/dt, where C is the current and Q the charge.

Such a current would be transient unless the conductors are connected with the terminals of a battery or ‘cell’ by which they are maintained at different potentials.

The presence of the current is manifested by heating of the wire and the creation of a magnetic field in its neighbourhood.

The field of a steady current is said to be a stationary field, because, although there is a flow of electricity, the conditions remain the same at all points as time progresses. We must point out however that directly the two conductors above are connected the field ceases to be electrostatic; we have therefore no longer any ground for asserting the existence of a single-valued potential function, and the fact that the field of a steady current is derivable from a potential function (not single-valued) is a new empirical assumption.

In the electron theory metals contain a number of electrons moving freely between the molecules. If there be no external electric field, the velocities of the free electrons are in random directions and there is no tendency for a collective motion in one direction rather than another.