New Television and Radio Services 143 



actual sound or picture travels through the wires or through 

 the air. Electric current travels through wires; radio waves 

 travel through space. The sound or picture which is to be 

 sent to some distant point by wire must first cause a varia- 

 tion in electric current. It is this variation in electric current 

 which is sent through the wire. Likewise, a sound or a pic- 

 ture which is to travel by radio waves through space must 

 cause a variation in those radio waves. It is this variation 

 which travels through space. 



When we talk into a telephone, we make sound waves that 

 travel through the air and vibrate against a metal disk, moving 

 it. This vibrating disk, in turn, affects electric current, which 

 varies in time with the vibrations of the disk. This varying 

 electric current travels through the telephone lines. At the 

 receiver on the other end of the line, the action is reversed. 

 Here the electric current (which is varying in time with the 

 disk against which we spoke) affects a second disk in the 

 receiver and sets it in motion. This second disk, vibrating in 

 time with the variations in the electric current (and conse- 

 quently in time with the first disk), starts sound waves through 

 the air. These sound waves which reach the ear of the listener 

 are the exact repetition of the sound waves which made the 

 first disk vibrate. 



The transmission of sound by radio is similar to the trans- 

 mission of sound by telephone. First the sound waves vibrate 

 against a microphone and set it in motion. This vibration of 

 the microphone causes electric current to vary in time with 

 the vibrations of the microphone. These variations in the elec- 

 tric current are used to vary or "modulate" the steady alter- 

 nating current which is alternating at the "frequency" on 

 which that transmitting station broadcasts. Most of our sta- 

 tions today use A.M., or "amplitude modulation," varying the 

 "size" of the wave. The P.M. stations use "frequency modu- 

 lation," varying the frequency of the wave. Whether A.M. 



