ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 267 



lator and its parabolic mirror are allowed to fall perpendicularly 

 upon a plane sheet of metal AB, as shown in Fig. 201, the 

 resonator is not acted upon if it is placed at certain points 

 n, n' , n' ', and so on, whereas the resonator is acted upon if it is 

 placed at positions intermediate between these points. The 



oscillator 



Waves 



Fig. 201. 



reflected waves from AB, Fig. 201, form with the advancing 

 waves a stationary wave train of which nodes are situated at the 

 points n, n', n" , and the antinodes at the points aa. 



146. The law of induced electromotive force and its bearing upon 

 electromagnetic wave motion. Let H be the intensity in gausses 

 of the magnetic field in the region of the wave shown in Fig. 195, 

 let f be the intensity of the electric field in abvolts per centi- 

 meter, and let / be the distance across from wire to wire (sheet 

 to sheet). The sidewise motion of the magnetic field at velocity 

 V induces an electromotive force in the region of the wave and 

 this electromotive force in abvolts is given by the equation 



as explained in Art. 64. Therefore the electric field intensity in 

 the wave (//) is given by the equation 



f=HV 



(78) 



in which f is expressed in abvolts per centimeter, H is ex- 

 pressed in gausses, and V is expressed in centimeters per 

 second. 



