226 PHYSICAL SCIENCE 



secondary of the induction coil, and, if these ends 

 be coupled back with the grid and the filament 

 in the right direction, the oscillations will give 

 to the grid the proper alternating potential to 

 maintain the oscillations and the plate circuit. 

 The apparatus is thus self-supporting when a 

 current is passed through it, and will continue to 

 produce oscillations, the period of which depends on 

 the induction of the coils and the electric capacity 

 of the system. By adjusting the induction and 

 capacity, the period of the oscillations can be tuned. 



Hence a thermionic valve may be used, both 

 to emit continuous waves and to rectify alternating 

 currents when received. By starting and stopping 

 such a train of waves at appropriate intervals, a 

 series of long and short signals may be emitted 

 at one station and received at another in the so- 

 called Morse code. 



Continuous waves may also be used to transmit 

 speech by telephony. The alternations of these 

 waves are much too rapid — in the neighbourhood 

 of a million a second — to pass through a telephone 

 or make it sound. But if the currents they produce 

 in an aerial wire be rectified, and then interrupted 

 from loo to 10,000 times a second, a sound of 

 corresponding pitch is heard in the telephone. 



That is the principle of broadcasting speech 

 or other sounds. A steady, continuous undamped 

 wave is emitted from the sending station. This 

 *' carrier wave" has some definite wave-length — 

 say 465 metres, which gives a frequency of 645,000 

 alternations per second. 



On this carrier wave, changes and interrup- 

 tions are superposed by speaking into a micro- 

 phone connected with the circuit. At the receiving 



