264 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



charges, positive on the lower wire and negative on the upper 

 wire, in Fig. 195. It is evident, furthermore, that the uniform 

 rotation of the ether cells in the region of the wave involves the 

 slipping of the ether cells where they come in contact with the 

 sheets of metal which bound the wave. This slipping constitutes 

 an electric current which flows to the left in the upper wire and 

 to the right in the lower wire in Fig. 195. 



Figure 195 represents what is called a rectangular wave pulse. 

 Fig. 196 shows what takes place when a simple train of electro- 

 magnetic waves travel along between two broad metal sheets. 



direction of progression 



\ 



0X18 



Fig. 196. 



Hcrttfs experiments with electric waves.* The oscillator used 

 by Hertz consisted of two brass rods A and B with an air gap 

 g, as shown in Fig. 197. These two rods were connected to 

 the terminals of an induction coil as indicated, at each impulse 

 of electromotive force from the induction coil a spark breaks 



* These experiments were described originally in Witdemann* s Annalen. A very 

 complete description of them may be found in Hertz's book on Electric Waves, Eng- 

 lish translation published by The Macmillan Company. 



