1890.] Prof. Fitz Gerald on Electromagnetic Radiation. 77 



WEEKLY EVENING MEETING, 



Friday, March 21, 1890. 



Sir James Criohton Browne, M.D. LL.D. F.R.S. Treasurer and 

 Vice-President, in the Chair. 



Professor G. F. Fitz Gerald, M.A. F.R.S. 



Electromagnetic Hadiation. 



In order to discover whether actions are propagated in time or in- 

 stantaneously, we may employ the principle of interference to measure 

 the wave-length of a periodic disturbance, and determine whether it 

 is finite or no. This is the principle employed by Hertz to prove 

 experimentally Maxwell's theory as to the rate of propagation of 

 electromagnetic waves. In order to confine the experiments within 

 reasonable limits we require short waves, of a few metres' length at 

 most. As the highest audible note gives waves of five or six miles 

 long, and our eyes are sensitive only to unmanageably short waves, it 

 is necessary to generate and observe waves whose frequency is inter- 

 mediate between them, of some hundred million vibrations per second 

 or so. For this purpose we may use a pair of conducting surfaces 

 connected by a shorter or longer wire, in which is interposed a spark- 

 gap of some few millimetres' length. When the conductors are 

 charged by a coil or electrical machine to a sufficiently high difference 

 of potential for a spark to be formed between them, they discharge 

 in a series of oscillations, whose period for systems of similar shape 

 is inversely proportional to the linear dimensions of the system so 

 long as the surrounding medium is unaltered. When the surrounding 

 non-conducting medium changes, the period depends on the electric 

 and magnetic specific inductive capacities of this medium. Two such 

 Systems were shown. A large one, whose frequency was about 

 60 millions per second ; and a small one, whose frequency was about 

 500 millions per second. The large one consisted of two flat plates, 

 about 30 cm. square and 60 cm. apart, and arranged in the same way 

 as is described by Prof. Hertz in Wiedemann's ' Annalen,' April 1888. 

 The smaller vibrating system consisted of two short brass cylinders 

 terminating in gilt brass balls of the same size, and arranged in the 

 same way as the smaller system described by Prof Hertz in Wiede- 

 mann's ' Annalen,' March 1889. This latter system was placed in 

 the focal line of a cylindrical parabolic mirror of thin zinc plate, 

 such as that described by Prof. Hertz in this paper. 



These generators of electromagnetic oscillations may be called 

 electric oscillators, as the electric charge oscillates from end to end. 

 A circle of wire, or a coil in which an alternating current ran, or, if 



