The Followers of Maxwell. 347 



average value of the radiant energy of electric type at distance 

 r from the oscillator is 2iT z A*S 2 /3c*r i T* per unit volume. The 

 radiant energy of magnetic type ma}' be calculated in a similar 

 way, and is found to have the same value ; so the total radiant 

 energy at distance r is 47r 3 ^4 2 /S^/3cVT 4 per unit volume; 

 and therefore the energy radiated in unit time is 16ir 4t A' i S 2 /3c s T*. 

 This is small, unless the frequency is very high ; so that 

 ordinary alternating currents would give no appreciable radia- 

 tion. Fitz Gerald, however, in the same year* 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 Fitz Gerald radiator constructed on this principle is 

 closely akin to the radiator afterwards developed with such 

 success by Hertz : the only difference is that in Fitz Gerald'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 electro- 

 magnetic field, which was discovered in 1884 by John Henry 

 Poynting.-)- 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 



* Brit. Assoc. Rep., 1883 ; FitzGerald's Scientific Writings, p. 129. 

 tPhil. Trans, clxxv (1884), p. 343. 



