836 The Outline of Science 



can therefore be led to flow through a circuit connected to the 

 cylinder. The current can only flow one way, namely, from the 

 hot filament to the cold metal cylinder, since the cylinder itself is 

 not giving off electrons. Such is the simplest principle of this 

 form of valve or detector, and owing to its sensitiveness and its 

 ease of adjustment, it is now, in one form or another, replacing all 

 other types of valves. If this thermionic valve is connected up to 

 the telephone in the place of the crystal rectifier, it will operate in 

 exactly the same way. 



The future of wireless telegraphy and telephony is full of 

 promise. "The matter of greatest interest at the present time," 

 says Professor J. A. Fleming, "is the remarkable developments 

 which have taken place in the thermionic valve, both as generator, 

 detector, and amplifier of electric oscillations. We are only at 

 the very beginning of this evolution, yet it has already completely 

 revolutionised the practical side of wireless telegraphy, as well as 

 telephony, with and without wires." 



3 



The waves used in wireless telegraphy vary in length accord- 

 ing to the purpose they are to serve. Thus for ordinary ship work, 

 communications between ship and ship or between ship and shore, 

 the wave-length used is round about 2,000 feet. Now an electro- 

 magnetic wave travels about 1,000 million feet in a second, so that 

 the time difference between the front and back of a wave 2,000 

 feet long is only 1/500,000 of a second. This, therefore, is the time 

 taken by one complete electric oscillation in the aerial. The 

 electrons, to produce these waves, must oscillate 500,000 times 

 per second. For long-distance wireless telegraph stations much 

 longer waves are employed they may be anything between 6,000 

 and 20,000 feet. The oscillations necessary to produce them are 

 correspondingly slower. Waves 20,000 feet in length, for in- 

 stance, only require 50,000 oscillations per second. 



We have seen that every wireless telegraph circuit is essen- 



