Theorem on oscillating power

The article tells about the contribution of Vainstein´s theorem in the development of electromagnetic theory. In accordance with this theorem the total power of electromagnetic field consists of active and oscillating components. The first magnitude is the active part of the power flow, equal to its average value. The second magnitude is the oscillating part of the power flow. It oscillates in time with a double frequency. An example of a problem, which has been solved by means of this theorem, - radiation of an antenna - is considered in the paper.