118 Prof. H. Hertz on the Propagation 



were of extraordinarily short period, it was convenient to 

 prove by means of these the accuracy of the inferences drawn. 

 In fact, the theory was proved by the experiments which will 

 now be described ; and it will be found that these few expe- 

 riments suffice to confirm in the highest degree the view of 

 Messrs. Heaviside and Poynting. Analogous experiments, 

 with similar results, but with quite different apparatus, have 

 already been made by Dr. 0. J, Lodge *, chiefly in the 

 interest of the theory of lightning-conductors. Up to what 

 point the conclusions are just which were drawn by Dr. Lodge 

 in this direction from his experiments, must depend in the first 

 place on the velocity with which the alterations of the elec- 

 trical conditions really follow each other in the case of 

 lightning. 



The apparatus and methods which are here mentioned are 

 those which I have described in full in previous memoirs f. 

 The waves used were such as had in wires a distance of nearly 

 3 metres between the nodes. 



1. If a primary conductor acts through space upon a 

 secondary conductor, it cannot be doubted that the effect 

 penetrates the latter from without. For it can be regarded as 

 established that the effect is propagated in space from point to 

 point, therefore it will be forced to meet first of all the outer 

 boundary of the body before it can act upon the interior of it. 

 But now a closed metallic envelope is shown to be quite opaque 

 to this effect. If we place the secondary conductor in such a 

 favourable position near the primary one that we obtain sparks 

 5 to 6 mm. long, and surround it now with a closed box made 

 of zinc plate, the smallest trace of sparking can no longer be 

 perceived. The sparks similarly vanish if we entirely surround 

 the primary conductor with a metallic box. It is well known 

 that, with relatively slow variations of current, the integral 

 force of induction is in no way altered by a metallic screen. 

 This is, at the first glance, contradictory to the present expe- 

 riments. However, the contradiction is only an apparent one, 

 and is explained by considering the duration of the effects. In 

 a similar manner, a screen which conducts heat badly protects 

 its interior completely from rapid changes of the outside tem- 

 perature, less from slow changes, and not at all from a con- 

 tinuous raising or lowering of the temperature. The thinner 

 the screen is the more rapid are the variations of the outside 

 temperature which can be felt in its interior. In our case also 

 the electrical action must plainly penetrate into the interior, if 



* Lodge, Journ. Soc. Arts, May 1888; Phil. Mag. [5] xxvi. p. 217 (1888). 

 t Hertz, Wied. Ann. xxxiv. p. 551 (1888). 



