128 METHODS OF COMMUNICATION FOR FOREST PROTECTION 



and for the size of the machine are extremely powerful and efficient generators of 

 electric currents. They produce true alternating currents at about 65 to 75 volts and 

 a frequency of 15 cycles per second, are very compactly built, and are not very 

 likely to give trouble. 



The two principal parts of the generator are the magnets and the armature. 

 The former are horseshoe-shaped and are made of a high quality of steel, and strongly 

 magnetized. Between the poles of the magnets is placed the armature. It consists 

 of an H-shaped laminated iron core around which is wound a coil of fine insulated 

 wire. A small crank with gear wheels is used to rotate the armature at a high rate 

 of speed between the poles of the magnets, thus producing the current. It will be 

 apparent, however, that since the generator serves only to produce current for ring- 

 ing purposes there is no reason why it should be permanently in the circuit. On the 

 other hand, there are certain objections to such permanent connection. The arma- 

 ture winding if permanently bridged across the line would form an additional path 

 to ground, thus reducing the current available for ringing other instruments. Also, 

 injury from outside currents might occur which can be avoided if the armature is 

 disconnected while it is at rest. For these reasons the magneto generators are usually 

 arranged with an automatic circuit-breaking device, by which the winding of the 

 armature is thrown out of the circuit whenever it is 'at rest and is thrown into the 

 circuit by the act of turning the generator crank. It will be noted that as soon as 

 the crank on the standard instrument is turned it slips inward a fraction of an inch. 

 This slight movement serves to close the circuit and throw the coil of the generator 

 armature on to the line, thus permitting the current from the. generator to flow out 

 over the line to ring the bells at the distant stations. 



6 KlNGER 



The low-frequency alternating current from the magneto generator operates a 

 polarized bell or ringer (Fig. 79) at each station by means of which the attention of the 



Fig. 79 Polarized ringer, unbiased type 



distant station is secured. Code ringing is employed on forest lines exclusively; that 

 is, each station has its own signal or call letter made by a combination of long and 



