160 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
in every way better to put it below ground, in other words, to employ 
an earth plate and compensate for any slight earth damping by an 
antenna of rather larger capacity. 
This matter is, however, only part of a much larger question, viz, 
the function of the earth in radiotelegraphy. It is well known that 
the nature of the earth’s soil or surface between the sending and 
recelving 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 
experimental researches of Admiral Sir Henry Jackson and the 
theoretical discussions of M. Brylinski and Doctor Zenneck. To fol- 
low their explanations 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 sub- 
stitute 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 com- 
prising an inductance and a capacity and some means for testing the 
amplitude of the oscillations set up in this secondary circuit. This 
decrease is not due to the mere fact that the iron has a greater resist-— 
ance than copper, but to the fact that the iron is magnetizable, and 
such magnetization absorbs 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 galvanize 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 zine from 
a piece of galvanized-iron wire, we render it a worse conductor for 
high-frequency oscillations. This experiment proves that such oscil- 
lations are conveyed by a thin surface layer of the conductor. In 
the case of a copper wire for oscillations having a frequency of one 
million, the current penetrates about one-third of a millimeter, and 
in the case of an iron wire, about one-fortieth of a millimeter into the 
metal. 
For nonmagnetic 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 farther into carbon than into metal. Accordingly 
a much thicker layer of carbon than of zinc would be needed to shield 
the iron spiral in our last experiment. The same thing happens in 
