1007.] on Recent Contributions to Electric Wave Telegraphy. 681 



At any distance from the antenna, and at any one spot, the 

 magnetic and electric forces are therefore periodically varying in 

 magnitude, and owing to the finite rate of propagation of the forces 

 through space we find that at certain e(|uispaced intervals these forces 

 are similarly reversed in direction at the same instant. 



When we speak of the length of the electric waves we mean the 

 shortest distance which separates two adjacent places at which either 

 the electric or magnetic force reverses direction in the same way at 

 the same instant. In wireless telegraphy the length of waves 

 employed may vary from 200 or iJOO feet to many thousands of feet 

 or several miles. The determination of this wave length is a 

 practically important matter, and accordingly instruments have been 

 designed specially for its measurement by Donitz, by Professor Slaby, 

 and by me. I have ventured to name my own appliance for 

 measuring long electric wave lengths, a cymometer.^ The importance 



c 



'^f^=-^s^:::d^ 



Fig. 3. 



Lines of Magnetic Force round an Antenna during Discharge. 



of the measurement is as follows : We know that the properties of 

 short electric waves constituting light and radiant heat depends upon 

 their wave length, and that some bodies are opaque to light waves but 

 transparent to heat waves. So in the case of the much longer ether 

 or electric waves used in telegraphy, the ease with which they pass 

 through buildings, forests, and even mountains or cliffs, or round the 

 earth's curved surface is determined by their wave length. Waves of 

 one or two hundred feet in length are considerably obstructed by the 

 closely packed houses in a town, but much longer waves go easily 

 through them. The measurement of the wave length is made to 

 depend upon the fact that there is a simple relation between the 

 velocity of these waves (which is the same as that of light), the 



* See Proc. Roy. Soc, vol. Ixxiv., p. 490, 1905. On an instrument for the 

 Measurement of the Length of Long Electric Waves. Also Phil. Mag., June, 

 1905, on the Applications of the Cymometer. 



