average value of the radiant energy of electric type at distance r from the oscillator is per unit volume. The radiant energy of magnetic type may be calculated in a similar way, and is found to have the same value; so the total radiant energy at distance r is per unit volume; and therefore the energy radiated in unit time is . This is small, unless the frequency is very high; so that ordinary alternating currents would give no appreciable radiation. FitzGerald, however, in the same year[1] indicated a method by which the difficulty of obtaining currents of sufficiently high frequency might be overcome: this was, to employ the alternating currents which are produced when a condenser is discharged.
The FitzGerald radiator constructed on this principle is closely akin to the radiator afterwards developed with such success by Hertz: the only difference is that in FitzGerald's arrangement the condenser is used merely as the store of energy (its plates being so close together that the electrostatic field due to the charges is practically confined to the space between them), and the actual source of radiation is the alternating magnetic field due to the circular loop of wire: while in Hertz's arrangement the loop of wire is abolished, the condenser plates are at some distance apart, and the source of radiation is the alternating electrostatic field due to their charges.
In the study of electrical radiation, valuable help is afforded by a general theorem on the transfer of energy in the electromagnetic field, which was discovered in 1884 by John Henry Poynting.[2] We have seen that the older writers on electric currents recognized that an electric current is associated with the transport of energy from one place (e.g. the voltaic cell which maintains the current) to another (e.g. an electromotor which is worked by the current); but they supposed the energy to be conveyed by the current itself within the wire, in much
