654 Professor J. A. Meming [June 4, 



ceiving stations has a great effect upon electric waves passing over it. 

 Various imperfect explanations were given of this action in early 

 days, but the basis for a better knowledge has been laid by the experi- 

 mental researches of Admiral Sir Henry Jackson and the theoretical 

 discussions of M. Brylinski and Dr. Zenneck. To follow their explana- 

 tions it must be borne in mind that high frequency electric currents as 

 used in radiotelegraphy are confined chiefly to the surface of conductors 

 by means of which they are conducted. Such a current does not 

 distribute itself uniformly over the whole cross-section of a wire 

 carrying it, but is confined to a thin skin or surface layer. This can 

 be proved by the following experiment. We take a copper wire spiral, 

 or loop, and make it part of a circuit in which a high frequency 

 current exists. If we measure in any way the current in that circuit 

 we find it has a certain value. If we substitute for the copper 

 wire an iron wire of the same size, we find that the current in the 

 circuit is then much less. This can be discovered by placing near 

 the circuit in question another testing circuit, comprising an inductance 

 and a capacity and some means for testing the amplitude of the oscil- 

 lations set up in this secondary circuit. This decrease is not due to 

 the mere fact that the iron has a greater resistance than copper, but 

 to the fact that the iron is magnetisable, and such magnetisation ab- 

 sorbs energy owing to so-called hysteresis. If, however, we dip the 

 iron for a moment into molten zinc and deposit on it a thin surface 

 layer of zinc, or galvanise it, we find it then becomes almost as good 

 as a solid copper wire for conveying high frequency currents. On the 

 other hand, if we burn off the zinc from a piece of galvanised iron 

 wire, we render it a worse conductor for high frequency oscillations. 

 This experiment proves that such oscillations are conveyed by a thin 

 surface layer of the conductor. In the case of a copper wire for 

 oscillations having a frequency of 1 milUon, the current penetrates 

 about ^ of a millimetre, and in the case of an iron wire, about 

 ^^o mm. into the metal. 



For non-magnetic substances the depth to which a current of a 

 given frequency penetrates into a conductor is greater in proportion 

 as the conductivity of the material is less. Hence high frequency 

 currents penetrate further into carbon tlian into metal. Accordingly 

 a much thicker layer of carbon than of zinc would l)e needed to 

 shield the iron spiral in our last experiment. The same thing 

 happens in the case of an electric wa^•e propagated over a terrestrial 

 surface. If the surface is a very good conductor, the wave hardly 

 penetrates into it, but glides over the surface. If it is a poor con- 

 ductor, the wave penetrates into it to a greater extent, and the worse 

 the conductivity the deeper the penetration. 



The materials of which the earth's crust is composed, with some 

 exceptions, owe their electric conductivity chiefly to the presence of 

 water in them. They are called electrolytic conductors. Substances 

 like marble and slate when free from iron oxide are fairly good insu- 



