REPETITION OF HERTZ'S EXPERIMENTS, 



AND DETERMINATION OF THE DIREC'ITON OF THE VIBRATION OF LIGHT.* 



By Frederick T. Trouton. 



Siuce last October (1888), Professor Fitzgerald and I have been re- 

 peating some of Professor Hertz's experiments, as occasion allowed, 

 and it may not be without interest at the present time to give a short 

 account of our work. 



The first experiment tried was the interference of direct electromag- 

 netic radiation with that retiected from a metallic sheet. This experi- 

 ment is analogous to that known in optics as " Lloyd's experiment." 



The radiation was produced by disturbances caused in the surround- 

 ing space by electrical oscillations in a conductor. It was arranged in 

 this wise. Two thin brass plates, about 40 centimeters square, were 

 suspended by silk threads at about 60 centimeters apart, so as to be in 

 the same plane. Each plate carried a stiff wire furnished at the end 

 with a brass knob. The knobs were abcut '.) millimeters apart, so that 



Fig. 1. 



on electrifying one plate a spark could easily pass to the other. This 

 spark, as is well known, consists not simply of a transference of half 

 the electricity of the first plate to the second — though this, which is 

 the final state, is all that is observable by ordinary experimental meth- 

 ods — but the whole charge passes across to the second plate, then re- 

 turns, and so on, pendulum-fashion, the moving part of the charge 

 becoming less each time, till finally brought to rest, the energy set free 

 at sparking being converted partly into heat in the wire and air break, 

 partly into radiation into space, or in terms of action at a distance in 

 inducing currents in other bodies. 



The time taken by the charge to pass over to the second i)late and to 

 return, is a definite thing for a given sized arrangement, and depends 

 on the connection between them. If be the capacity of the plates, 

 and I the self-induction of the connection, the time of each complete 

 oscillation equals 27C\/ (01). The time in the case of the particular ar- 



* From ^'(ttllr(■, Feb. 21, 1H81», vol. xxxix, pp. :]01-39:{. 



191 



