124 Prof. H. Hertz on the Propagation 



from A towards 8. We thus obtain brilliant sparks at A ; 

 they are of similar intensity to those obtained when the wire 

 was inserted without any screen. The sparks do not become 

 materially smaller, if, without making any other alteration, 

 we lengthen the tube 7 considerably, even to 4 metres. 

 According to the usual theory it would be said that the wave 

 arriving at A penetrates easily the thin, good-conducting 

 metal disk a, then it leaps across the spark-gap at A, and 

 travels on in the central wire. According to our view, on 

 the contrary, we must explain the phenomenon in the follow- 

 ing manner. The wave arriving at A is quite unable to 

 penetrate the metallic disk ; it therefore glides along the disk 

 over the outside of the apparatus and travels as far as the 

 point 8, 4 metres away. Here it divides : one part, which 

 does not concern us at present, travels on immediately along 

 the straight wire, another bends into the interior of the tube 

 and then runs back in the space between the tube and the 

 central wire to the spark-gap at A, where it now gives rise 

 to the sparking. That our view, although more complicated, 

 is still the correct one, is proved by the following experiments. 

 Firstly, every trace of sparking at A disappears as soon as 

 we close the opening at 8, even if it be only by a stopper of 

 tinfoil. Our waves have only a wave-length of 3 metres ; 

 before their effect has reached the point 8 the effect at A has 

 passed through zero and changed sign. What influence then 

 could the closing of the distant end 8 have upon the spark at 

 A, if the latter really happened immediately after the passage 

 of the wave through the metallic wall ? Secondly, the sparks 

 disappear if we make the central wire terminate inside the 

 tube 7, or at the opening 8 itself ; but they reappear when we 

 allow the end of the wire to project even 20 to 30 centim. only 

 beyond the opening. What influence could this insignificant 

 lengthening of the wire have upon the sparks in A, unless the 

 projecting end were just the means by which a part of the 

 wave breaks off and penetrates through the opening 8 back 

 into the interior ? Thirdly, we insert in the central wire 

 between A and 8 a second spark-gap B, which we also com- 

 pletely cover with a gauze cage like that at A. If we make 

 the distance of the terminals at B so great that sparks can no 

 longer pass across, it is also no longer possible to obtain 

 visible sparks at A. But if we hinder in like manner the 

 passage of the spark at A, this has scarcely any influence on 

 the sparks in B. Therefore, the passage of the spark at B 

 determines that at A, but the passage of a spark at A does not 

 determine that at B. The direction of propagation in the 

 interior is therefore from B towards A, not from A to B. 



