New Television and Radio Services 145 



and you "modulated" a carrier wave with these variations, 

 you could transmit the variations of dots in the picture just 

 as you transmitted the variations of the sound. If your receiver 

 contained a device which was in time with, or "synchronized" 

 with, the back-and-f orth movement of the beam that scanned 

 the picture, you could repeat the picture on a screen in your 

 receiver, a dot at a time, just as it was picked up from the 

 transmitter, a dot at a time. 



If you have read the chapter on electronics, you will recall 

 the photocell a vacuum tube which contains a material 

 that will emit electrons when light falls on it. The icono- 

 scope, a tube that is used in television to transmit pictures, 

 contains a screen made up of thousands of tiny photocells, 

 each no larger than the dot in the photograph you looked at 

 in the newspaper. The picture to be transmitted is focused 

 on the screen, just as you focus a picture on the film in your 

 camera. As the picture is focused on the screen, the tiny 

 photocells emit electrons the number of electrons each cell 

 emits depending on the brilliance of the light or the density of 

 the shadow which is focused on it. The more electrons a cell 

 emits, the more positive becomes its charge. 



We might compare the screen of cells at this point with a 

 newspaper picture and say that these little cells, with the 

 variations in positive charge that they have, are the electric 

 black dots and white spaces of a newspaper picture. 



Then from an "electron gun" a stream of electrons scans 

 the picture, a row at a time, and each little cell reacts in 

 accordance with the amount of positive charge it now has. 

 The variation of each cell is used to modulate the carrier 

 wave which is transmitted from the television broadcasting 

 station, just as the variations in electric current from the 

 microphone vary the carrier wave for a sound broadcast. 



At the receiver another electron gun flashes back and forth 

 across a screen which "fluoresces" or lights up in accordance 



