THE MAGNETO TELEPHONE 127 



distance transmission. In brief, it accomplishes three things, all important to suc- 

 cessful telephone operation: 



(1) It produces true alternating currents in place of the direct current of vary- 

 ing intensity produced by the transmitter and battery alone. 



(2) It produces currents of much higher voltage (E.M.F.) than it is practicable 

 to secure from voltaic cells operating through carbon transmitters alone. 



(3) By limiting the circuit over which the current from the battery flows, to 

 the very short one including only the battery itself, the transmitter, and the primary 

 of the induction coil, and by making the last of low resistance, the variations in 

 resistance produced by the varying pressure in the transmitter under the influence 

 of sound vibration are in comparison to the entire resistance quite large. These 

 impulses are impressed upon the secondary current with an increase in strength and 

 amplitude corresponding to the increased pressure of that current, and are success- 

 fully transmitted out on the line to the receiving stations over very great distances. 



The telephone circuit as produced by including in it an induction coil is shown 

 in Fig. 77. This shows diagrammatically the circuit of the speech-transmitting por- 

 tion of the telephone substantially as it exists to-day. The rest of th^ telephone set 

 is required for signalling and is only incidental to the portion employed in trans- 

 mitting the sounds of the voice. 



5 MAGNETO GENERATOR 



The generator is simply a small dynamo. It differs from the large dynamos 

 used in generating power and light currents only in having permanent steel mngnets 





Fig. 78 Magneto generator, five bar 



in place of the usual electro-magnets. Generators in modern telephones are of 

 various sizes, depending on the kind of service for which they are intended. The 

 largest and most powerful are built with five magnets (five-bar generators) and are 

 the type used in the Forestry Branch standard instruments. They are intended to 

 ring through very high resistances, such as 'are encountered on forest telephone lines, 



