ELECTRIC WAVE TELEGRAPH Y—FLEMING. 167 
over any large distances can not be conducted unless the antenna is 
so placed that the electric force is perpendicular to the surface of the 
earth or sea. 
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 equispaced 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 em- 
ployed may vary from 200 to 300 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 Dénitz, by Professor Slaby, and by 
me. I have ventured to name my own appliance for measuring long 
electric wave lengths, a cymometer.. The importance of the measure- 
ment is as follows: We know that the properties of short electric 
waves constituting light and radiant heat depend upon their wave 
length, and that some bodies are opaque to hght waves but trans- 
parent 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 de- 
pend upon the fact that there is a simple relation between the velocity 
of these waves (which is the same as that of light), the periodic time 
of the oscillations in the antenna, and the wave length as expressed 
by the formula wave length = velocity < periodic time. Since the 
velocity is nearly 1,000 million feet per second, the wave length in feet 
is easily found, when we know the time period of the oscillations in 
the antenna. This last quantity can be found by placing near to the 
antenna a circuit in which secondary electric oscillations can be 
sympathetically induced by those in the antenna. For this purpose 
we must have a circuit which possesses the two qualities of capacity 
and inductance. This is secured by joining in series some form of 
Leyden jar or condenser and some form of spiral wave or inductance. 
Moreover, we must have the means of varying this capacity and in- 
ductance, so as to bring the cymometer circuit into tune, as it is called, 
“See Proc. Roy. Soec., Vol. LX XIV, 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. 
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