jo Book of Engineering 



magnet is a bobbin of very fine wire, the 

 ends of which are joined to the ends of the 

 two line wires. If you look at the next 

 telephone line you pass you will notice 

 that there are generally two lines for each 

 service, and that they cross each other on 

 their journey from pole to pole to obviate 

 what is called induction. 



When the telephone is used the sound of 

 the voice enters the transmitter, the sound 

 waves strike upon the disc, and so causes 

 it to vibrate, and the vibrations follow 

 exactly the sound waves. These are carried 

 along by means of the magnetic current, 

 flow along the line, and are then reproduced 

 on the microphone. 



Those are the general principles of the 

 working of the transmitter, line and receiver. 

 Now we can get outside again for a moment 

 and look at the lines. These are of copper, 

 though there were many steel lines in exis- 

 tence. Though galvanised, steel wires 

 quickly rust, and there is a loss of effective- 

 ness. Now, as far as possible, the trunk 

 routes (and also City lines) are being put 



