120 Prof. H. Hertz on the Propagation 



by the help of this wire. The usual theory is that a wave 

 travels along the wire in this case. But we shall try to show 

 that all the alterations are confined to the space outside and 

 the surface of the wire, and that its interior knows nothing of 

 the wave passing over it. I arranged experiments first of all 

 in the following manner. A pie"ee about. 4 metres long was 

 removed from the wire conductor and replaced by two strips 

 of zinc plate 4 metres long and 10 cm. broad, which were laid 

 flat one above the other, with their ends permanently connected 

 together. Between the strips along their middle line, and 

 therefore almost entirely surrounded by their metal, was laid 

 along the whole 4 metres length a copper wire covered with 

 gutta-percha. It was immaterial for the experiments whether 

 the outer ends of this wire were in metallic connexion with, 

 or insulated from, the strips ; however, the ends were mostly 

 soldered to the zinc strips. The copper wire was cut through 

 in the middle, and its ends were carried, twisted round each 

 other, outside the space between the strips to a fine spark-gap, 

 which permitted the detection of any electrical disturbance 

 taking place in the wire. When waves of the greatest possible 

 intensity were sent through the whole arrangement, there was 

 nevertheless not the slightest effect observable in the spark- 

 gap. But if the copper wire was then displaced anywhere a 

 few decimetres from its position, so that it projected just a 

 little beyond the space between the strips, sparks immediately 

 began to pass. The sparks were the more intense according 

 to the length of copper wire extending beyond the edge of the 

 zinc strips and the distance it projected. The unfavourable 

 relation of the resistances was therefore not the cause of the 

 previous absence of sparking, for this relation has not been 

 changed ; but the wire being in the interior of the conducting 

 mass, was at first deprived of the influence coming from 

 without. Moreover, it is only necessary for us to surround 

 the projecting part of the wire with a little tinfoil in metallic 

 communication with the zinc strips, in order to immediately 

 stop the sparking again. By this means we have brought the 

 copper wire back again into the interior of the conductor. If 

 we bend another wire into a fairly large arc round the pro- 

 jecting portion of the gutta-percha wire, the sparks will be 

 likewise weakened ; the second wire takes off from the first a 

 certain amount of the effect due to the outer medium. Indeed, 

 it may be said that the edge of the zinc strip itself takes away 

 the induction from the middle of the strip in a similar manner. 

 For if we now remove one of the strips, and leave the insulated 

 wire simply resting on the other one, we certainly obtain 

 sparks continuously in the wire ; but they are extremely weak 



