of Electric Waves through Wires. 119 



we only diminish sufficiently the thickness of the metal. But 

 I did not succeed in attaining the necessary thinness in a 

 simple manner ; a box covered with tinfoil protected com- 

 pletely, and even a box of gilt paper, if care was taken that 

 the edges of the separate pieces of paper were in metallic 

 contact. In this case the thickness of the conducting-metal 

 was estimated to be barely ^ mm - -"- now fitted the pro- 

 tecting envelope as closely as possible round the secondary 

 conductor. For this purpose its spark-gap was widened to 

 about 20 mm., and in order to detect electrical disturbances 

 in it an auxiliary spark-gap was added exactly opposite the 

 one ordinarily used. The sparks in this latter were not so 

 long as in the ordinary spark-gap, since the effect of resonance 

 was now wanting, but they were still very brilliant. After 

 this preparation the conductor was completely enclosed in a 

 tubular conducting envelope as thin as possible, which did not 

 touch it, but was as near it as possible ; and in the neighbour- 

 hood of the auxiliary spark-gap (in order to be able to use it) 

 the envelope contained a wire-gauze window. Between the 

 poles of this envelope brilliant sparks were produced, just as 

 previously in the secondary conductor itself ; but in the 

 enclosed conductor not the slightest electrical movement could 

 be recognized. The result of the experiment is not affected 

 if the envelope touches the conductor at a few points ; the 

 insulation of the two from each other is not necessary in order 

 to make the experiment succeed, but only to give it the force 

 of a proof. Clearly we can imagine the envelope to be drawn 

 more closely round the conductor than is possible in the expe- 

 riment ; indeed, we can make it coincide with the outermost 

 layer of the conductor. Although, then, the electrical dis- 

 turbances on the surface of our conductor are so powerful that 

 they give sparks 5 to 6 mrn. long, yet at ^ mm. beneath the 

 surface there exists such perfect freedom from disturbance that 

 it is not possible to obtain the smallest sparks. We are brought, 

 therefore, to the conclusion that what we call an induced 

 current in the secondary conductor is a phenomenon which is 

 manifested in its neighbourhood but to which its interior 

 scarcely contributes. 



2. One might grant that this is the state of affairs when the 

 electric disturbance is conve} r ed through a dielectric, but 

 maintain that it is another thing if the disturbance, as one 

 usually says, has been propagated in a conductor. Let us 

 place near one of the end plates of our primary conductor a 

 conducting-plate, and fasten to it a long straight wire ; we 

 have already seen in the previous experiments how the effect 

 of the primary oscillation can be conveyed to great distances 



