838 The Outline of Science 



of the length transmitted just as a violinist tunes his violin to the 

 pitch of an accompanying piano. 



Wireless Telephony 



The principle on which wireless telephony is based will be 

 easily understood by anyone who has followed the account given 

 here of wireless telegraphy. Sound, as we know, consists of air 

 waves, and those waves take the form of a to and fro movement 

 of the particles of air. Now when we speak into an ordinary 

 telephone the air waves set up by our voice causes a thin plate 

 the diaphragm to follow the air movements ; it bends in and out 

 in a manner corresponding to the to and fro motion of the air. 

 Behind the diaphragm a number of little carbon granules are 

 packed and, as the diaphragm moves, these granules are more or 

 less compressed together. Now such an arrangement of carbon 

 granules is a conductor of electricity, but its electrical conductivity 

 varies according to the degree the granules are compressed to- 

 gether. A current flowing through the granules varies in strength, 

 therefore, according to the movement of the diaphragm, that is, 

 according to the air waves being set up by the speaker's voice. 

 This varying current is conveyed along a wire and made to 

 operate a diaphragm at the other end, producing similar air waves 

 to the original ones and thus reproducing, very nearly, the words 

 spoken at the other end. 



In wireless telephony we dispense with the connecting wire. 

 The sending station sends out trains of undamped waves, which 

 are continuous waves of constant amplitude. These would not, 

 by themselves, affect the receiving telephone, since the oscillations 

 used are too rapid. But by incorporating a telephone transmitter 

 in the sending circuit and making the oscillations first flow through 

 this we can, by speaking into the transmitter, vary the strength 

 of the oscillations sent out in just the same manner as we vary 

 the strength of the current in the ordinary wire telephone circuit. 



