ELECTRICAL WAVES. 511 



The formula which expresses the relations is one from Lorenz 

 (" Annalen der Physik und Chemie," vii, p. 161) : 



A 



where T = time of oscillation of the electrical wave, P = the self- 

 induction of the conductor concerned, C = its electrostatic ca- 

 pacity, and A = velocity of electrical propagation, which is as- 

 sumed to be that of light. It will thus be seen that each conductor 

 has its own proper time of electrical oscillation and wave-length. 



If, now, the capacity of one side of the rectangle be increased, 

 the time of oscillation of the waves on that side will be also in- 

 creased. This will increase the wave-length, and equilibrium can 

 be established by adding the same capacity to the other side, or 

 by changing the point of contact. 



For the reason that the only variables in the time of oscilla- 

 tion are the self-induction and the capacity, the resistance and 

 material of the rectangle have no influence on the phenomena. 

 Because the capacity of each half of the rectangle is chiefly that 

 of the balls at its terminals, the employing of fine wire for one 

 half can produce no noticeable effect. 



That the size of the rectangle should have such an influence is 

 to be expected up to certain limits — that is, until the total length 

 of the sides is one wave-length or a multiple of the same. Then 

 the waves could be made to arrive at the terminals in opposite 

 phases, and would give the largest sparks. 



Were this the only proof which Hertz could give of interfer- 

 ence, a great deal of doubt might be cast upon its conclusiveness. 

 Would not one naturally expect that, if both sides of the rectangle 

 were of the same length and had the same capacity, the potential 

 on both balls would be the same, and no discharge could take 

 place ; or, when of different capacities, the charging and discharg- 

 ing following each other so rapidly that the same quantity of 

 electricity would tend to pass through a section of each side of 

 the rectangle, and would thus necessitate a discharge ? 



But Hertz's quantitative experiments are more satisfactory. 

 In order to understand them, a few preliminary phenomena must 

 be described. These relate to what he calls the principle of " reso- 

 nance." As any sound resonator, having its own proper wave- 

 length, can be set in vibration by a vibrating body of the same or 

 multiple time of vibration, so we might suppose that any electri- 

 cal conductor could be set in vibration by a neighboring electrical 

 wave disturbance of proper time of oscillation. This supposition 

 is verified by experiment. 



The apparatus and arrangement are very similar to those in 

 the previous experiment. However, instead of the two outer brass 

 balls on the Kuhmkorff discharger, two hollow zinc spheres of 



