122 Prof. H. Hertz on the Propagation 



at which it was connected to a conducting-tube, 7, which, 

 insulated from the central wire, surrounded it completely for 



Fig. 1. 



iT 



a length of 1*5 metre. The free end of the tube, S, was then 

 connected with the central wire. The wire, together with its 

 spark-gap, is once more situated in a metallically protected 

 space ; and it was only to be expected, from the previous 

 experiments, that not the slightest electrical disturbance would 

 be detected in the wire in whichever direction waves were sent 

 through the apparatus. So far, then, this arrangement pro- 

 mises nothing new, but it has the advantage over the previous 

 one that we can replace the protecting metallic tube, 7, 

 by tubes of smaller and smaller thickness of wall, in order to 

 investigate what thickness is still sufficient to screen off the 

 outside influence. Very thin brass tubes, tubes of tinfoil and 

 Dutch metal proved to be perfect screens. I now took glass 

 tubes which had been silvered by a chemical method, and it 

 was then perfectly easy to insert tubes of such thinness that, 

 in spite of their protecting power, brilliant sparks occurred in 

 the central wire. But sparks were only observed when the 

 silver film was no longer quite opaque to light and was cer- 

 tainly thinner than t -J-q mm. In imagination, although not 

 in reality, we can conceive the film drawn closer and closer 

 round the wire, and finally coinciding with its surface ; we 

 should be quite certain that nothing would be radically altered 

 thereby. However actively, then, the real waves play round 

 the wire, its interior remains completely at rest ; and the 

 effect of the waves hardly penetrates any more deeply into the 

 interior of the wire than does the light which is reflected from 

 its surface. For the real seat of these waves we ought not to 

 look, therefore, in the wire, but rather to assume that they take 

 place in its neighbourhood ; and instead of asserting that our 

 waves are propagated in the wire, we should be more accurate 

 in saying that they glide along on the wire. 



Instead of placing the apparatus just described in the cir- 

 cuit in which we produced waves indirectly, we can insert it in 

 one branch of the primary conductor itself. In such experi- 

 ments I obtained results similar to the previous ones. Our 

 primary oscillation, therefore, takes place without any partici- 

 pation of the conductor in which it is excited, except at its 



