12 Effect' of Conductors in Neighbourhood of a Wire. [May 2, 



surrounded by tin-foil, connected at both ends with the earth, were 

 fastened close to the balls of the resonator ; the other extremities of 

 these wires could move along the wires LM, NO respectively. When 

 the coil was working, sparks passed between the balls of the reso- 

 nator, and it was found that the intensity of these sparks depended 

 on the position of the points P and Q, to which the extremities of the 

 wires of the resonator were attached. The experiments made to 

 determine the velocity through the wire were as follows : the end Q 

 of one of the wires of the resonator was placed at 0, the end of the 

 wire NO and the extremity P of the other wire moved along LM 

 until the sparks in the resonator were as faint as possible ; the dis- 

 tance P^, when this was the case, was about 5 metres. We may 

 conclude that in this position the points V 1 and are nearly at the 

 same potential. The end of the other wire was then moved along NO 

 until the sparks were again as faint as possible ; the position Qj, when 

 this was the case, was such that Q T was between 10 and 10'25 metres. 

 Since the sparks are again a minimum, we may conclude that Pj and 

 Qj are again at nearly the same potential, hence the potentials at Q x 

 and O must be very nearly equal, but when this is the case, Q T 

 must be very nearly a wave-length ; the wave-length in the wire LM 

 was found in a similar way to be also about 10 metres. Hence the wave- 

 length of the electrical vibration in the wire must in this case be 

 about 10 metres, but Hertz has shown by the interference of the direct 

 electrical waves, and those reflected from a large metal reflector, that 

 the wave-length of the action propagated through the air from this 

 vibrator is also about 10 metres, and the length of the wave must be 

 approximately the same in our experiments as the resonator which 

 responded to the vibrations was of the same dimensions. So that 

 in this case the velocity of propagation through the wire is the same 

 as that through the air. 



Since the sparks between the balls of the resonator never actually 

 vanish, the determination of the places where they are as faint as 

 possible is a matter of judgment, and thus the method is not capable 

 of any very great accuracy. I found, however, on comparing my 

 results with those of another observer, Mr. E. Everett, that the two 

 sets agreed within about 2 feet in 10 metres. 



The rate at which the disturbances die away was determined by 

 a preliminary experiment. In this only one wire was used, and this 

 was carried over the laboratory until when the resonator was used in 

 the way described by Hertz no sparks passed between the balls ; the 

 length of wire necessary for this was more than twice as great for 

 copper as for German silver wire of the same diameter. Thus the rate 

 of decay depends on the material of which the wire is made, and, 

 therefore, by the above investigation the velocity through the wire is 



