Knowledge. 



With which is incorporated Hardwicke's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb, F.L.S., and E. S. Grew, M.A. 



MARCH, 1913. 



ELECTRIC WAVES. 



By W. D. EGGAR, M.A. 



Middle-aged people, like the writer, may still be 

 found who will answer, when asked the meaning of 

 the word electron, that it is the Greek name for 

 amber. This is admittedly 

 true, but it will probably be 

 unsatisfactory to the ques- 

 tioner, who will be expecting 

 much more up-to-date infor- 

 mation. Modern text-books, 

 if we may include anything 

 published within the last 

 twenty years as modern, will 

 suggest many different defin- 

 itions of the sub-atomic 

 bodies on which Dr. Johnstone 

 Stoney, with a somewhat mis- 

 placed generosity, bestowed 

 the title of electron. They 

 are centres of strain in the ether, cathode rays, 

 /3-particles, bricks of which atoms are built, ends of 

 Faraday tubes. The mass 

 of an electron is given as 

 6- 1 X 10 ~ 2M grammes, and 

 its radius as 10 ~ 13 centi- 

 metres, and some persons 

 with a taste for figures 

 may feel some small satisfaction in this information. 

 It is still open to anybody to form any conception 

 that he may find possible 

 of the ultimate structure 

 of space ; but certain 

 facts about the aether are 

 becoming part of our com- 

 mon life, and are even a 

 subject of inquiry for a 

 Parliamentary Com- 

 mittee. Whether we regard an electron as a whirl- 

 pool in a continuous medium, or as the terminus of 



Figure 71 



Figure 72. 



a Faraday tube in an aether of which the structure 

 is fibrous, there seems to be no question that move- 

 ments of electrons are accompanied by a disturb- 

 ance of the adjacent medium, 

 and that this disturbance 

 travels along the medium, 

 whatever its structure, with 

 a velocity of 3X10 10 centi- 

 metres, or approximately one 

 hundred and eighty-six 

 thousand miles per second. 



Wireless telegraphy is 

 accomplished by a succession 

 of electric, or electro-mag- 

 netic disturbances, propagated 

 as waves through space. It is 

 remarkable that Britain had 

 furnished the theory of these 

 waves before their existence was demonstrated 

 practically on the Continent. Sir J. J. Thomson, 



in " The Encyclopaedia 

 B r i t a n n i c a " ( X 1 1 h 

 Edition), points out that 

 Lord Kelvin, in 1853, 

 proved from theory that 

 the discharge of a Leyden 

 jar must be oscillatory. Feddersen proved it by 

 experiment. Clerk Maxwell proved that, on his 



theory, electro -magnetic 

 waves must travel through 

 space with the velocity 

 of light. Hertz, in 1887, 

 demonstrated the exist- 

 ence of these waves, em- 

 ploying as the source of 

 disturbance an oscillatory 

 He found that such a discharge 

 small sparks between the ends 



spark-discharge, 

 would produce 



81 



