The two great branches of electrical theory which concern the phenomena of electricity at rest, or " frictional " or " static " electricity, and of electricity in motion, or electric currents, are treated in two separate articles, Electrostatics and Electrokinetics.
The fundamental fact connecting electric currents and magnetic fields is that the line integral of magnetic force taken once round a conductor conveying an electric current is equal to 4 7r-times the surface integral of the current density, or to 4 7r-times the total current flowing through the closed line round which the integral is taken (see Electrokinetics).
Such an arrangement constitutes in effect a condenser, and when the two plates respectively are connected to the secondary terminals of an induction coil in operation, the plates are rapidly and alternately charged, and discharged across the spark gap with electrical oscillations (see Electrokinetics).