186 HERTZ'S EESEARCHES ON ELECTRICAL WAVES. 



portion wlrich \v:is metallic was not of any sreat extent. The E. m. f. 

 would tbeiefore Lave a finite value at its surface, and would be in the 

 direction of the impinging waves. The node, which in the case of perfect 

 conductivity would occur at the surface of the wall, would therefore 

 actually be situated a little behind it, as shown at A in the diagram. 

 If then twice the distance A B — that is to say, the distance A C — is 

 half the wave-length the steady waves will be as represented by the 

 continuous lines in Eig. 12. The E. M. F.'s acting on each side of the 

 circles, in the i)ositions I, II, III, and IV, will therefore at a given 

 moment be represented in magnitude and direction by the arrows on 

 each side of them in the diagram. If therefore in the neighborhood 

 of a node the air space is turned towards the node, the strongest e. m. f. 

 in the circle will act under more favorable conditions against a weaker 

 one under less favorable conditions. If however the air space is turned 

 away from the node, the stronger e. m. f. acts under less favorable con- 

 ditions against a weaker one under more favorable conditions. In the 

 latter case the resultant action must be less than in the former, which- 

 ever of the two E. M. F.'s has the greater effect, which explains the 

 change of sign of the phenomenon at each quarter wave-length. 



This explanation is further confirmed by the consideration that if it is 

 the true one, the change of sign at the points B andD must take place 

 in quite a different manner froui that of the point C. The e. m. f.'s, 

 acting on the secondary circle, in the positions V, VI, and VII, are 

 shown by the corresponding arrows, and it is clear that in the positions 

 B and D, if the air space is turned from one side to the other, the vibra- 

 tion will change its direction round the circle, and.therefore the spark- 

 ing must during the rotation vanish either once or an uneven number 

 of times. In the position 0, however, the direction of vibration remains 

 unaltered, and therefore the sparks must disappear an even number of 

 times, or not at all. 



The experiments showed that at B and T) the sparking diminished as 

 the air space receded from a, vanished at the highest point, and again 

 attained its original value at the point fJ. At C, on the other hand, 

 the sparking continued throughout the rotation, being a little stronger 

 at the highest and lowest points. If then there is anj^ change of sign 

 in the position C, it must occur with very much smaller displacements 

 than in other positions, so that in any case there is a distinction such 

 as required between this and the other two cases. 



Another very direct proof of the truth of Dr. Hertz's presentation of 

 the nature of the waves was obtained. If the secondary circle lies in 

 the jdane of the waves instead of in the plane of vibration, the e. m. f. 

 must be equal at all j)oints of the circle, and for a given position of the 

 air space, the sparking must be directly proportional to its intensity. 

 When the experiment was made it was fbnnd, as expected, that at all 

 distances the sparking vanished at the highest and lowest points of the 

 circle, and attained a maximum value at the points in the horizontal 

 plane through the point of incidence. 



