IV. On an instrument for indicating and measuring difference of phase between E. M. F. and current in any alternating current system
P Cardew · Proceedings of the Royal Society of London · 1894
Abstract If the periodic time of an alternating E. M. F be T, and if, owing to the presence of capacity or self-induction, or both in the circuit, the current passes through the value 0 at times differing from the times of passage of the E. M. F. through the same value by t, the electrical power will be V × C × cos (2πt/T), where V indicates the effective volts and C the effective current.