274 OUR PHYSICAL WORLD 



Several turns of the antenna inductance are shorted by large 

 relays. These relays are actuated by a current which can be 

 controlled by the telegraph key or by some mechanical sending 

 device. The effect of shorting a portion of the antenna induc- 

 tance is to change the frequency of the transmission wave at 

 intervals, corresponding to dots and dashes. The result at the 

 receiving station will be a succession of notes at two different 

 pitches which can readily be interpreted by the receiving opera- 

 tor into the dots and dashes of the telegraph code. If the tuning 

 of the receiving station is sufficiently accurate, the only note 

 heard will be the one caused by the frequency produced when the 

 key at the sending station is closed. The wave which is sent out 

 by the transmitting station when the key is not depressed is 

 called the compensating wave. Very accurate tuning at the 

 receiving end is necessary to tune out this wave. Later practice 

 has been to ground this compensating wave through the water- 

 cooling system of the tube so that it does not cause confusion at 

 the receiving station. 



It remains now to explain how speech and music may be 

 sent out by radio. The principle of the radio telephone trans- 

 mission is fundamentally the same as the principle of continuous- 

 wave transmission, with the addition of some means of impressing 

 on the continuous wave the sound or audio-frequency modula- 

 tion. This modification is made, not in the frequency of the 

 transmitting wave, but in its current strength or amplitude. 



This impressing of the speech wave upon the continuous wave 

 is known as voice modulation, and is shown in the diagram of 

 Figure 127. The continuous wave in this case is called the carrier 

 wave. Its frequency is very high, between five hundred thousand 

 and one million double vibrations per second. This high fre- 

 quency is necessary in order that the voice tones, with their 

 varying frequencies of around five hundred to one thousand 

 double vibrations per second, may be faithfully reproduced. 

 Thus each wave of the sound will be outlined by the increasing 

 and decreasing amplitudes of about one thousand radio waves. 



