Applied Science 835 



passed direct through the telephone it would not operate, as the 

 currents pass too rapidly, first in one direction and then in the 

 other, to produce any resultant effect. For this reason an electric 

 valve must be included in the receiving circuit, that is to say, some 

 device which enables the current to pass in one direction but not in 

 the other. There are several such devices; a well-known one is 

 the crystal rectifier. Many kinds of crystal can be used, some of 

 the better known being carborundum, zincite, iron pyrites, and 

 bornite. These crystals have the peculiar property of allowing 

 electric currents to flow through them more easily in one direction 

 than in the other. If one of these crystals be connected up to the 

 telephone, therefore, the oscillations arriving at the receiving 

 aerial are, as it were, weeded out, so that only those currents which 

 flow in one direction are allowed to pass. 



A train of waves mounts up and produces a click in the 

 telephone. A short series of these trains produces a short musical 

 note; a longer series of trains produces a longer note. In this 

 way the receiving operator can hear the dot and dash signals sent 

 from the transmitting station, and consequently can receive in- 

 telligible messages. 



We have seen that an essential part of the receiving ap- 

 paratus is an electric valve, a device for enabling the received cur- 

 rent to flow in one direction but not in the other. The most 

 important and most extensively employed of these valves, the 

 thermionic valve, has revolutionised wireless telegraphy of late; 

 this valve takes advantage of the fact that electrons are emitted 

 from a hot filament. We shall describe this valve in its simplest 

 form. Inside an ordinary electric light bulb there is a metal 

 cylinder surrounding the filament, but nowhere touching it. Now 

 let us see what happens when the filament is heated. A number 

 of electrons escape from the surface of the filament. In the 

 ordinary way they would simply fall back on it. But if the 

 surrounding metal cylinder is positively charged it will attract the 

 negatively charged electrons, and they will pass over to it and 



