130 



METHODS OF COMMUNICATION FOR FOREST PROTECTION 



7 SWITCH-HOOK 



It has beni previously indicated that there are two entirely different sources of 

 electric current in a magneto telephone set. The one is the local battery; the other is 

 the magneto generator. The current from the local battery traverses a very short circuit 



Fig. 80 Telephone switch-hook 



through transmitter and primary of the induction coil only, but induces a secondary 

 current in the induction coil which traverses the whole line, thus enabling conversation 

 to be carried on. The current from the magneto generator flows out over the line, 

 also, and actuates the bells at all other stations for signalling purposes. When the 

 instrument is not being used it is obviously undesirable to have the parts used for 

 talking connected to the line. If this continuous connection existed, conversation 

 between any two stations would be heard at all other stations and as part of the 

 talking current would pass through each receiver connected to the line the amount 

 received by any one of them would be proportionately reduced. The leakage through 

 the low resistance of the numerous receivers and induction coils, moreover, would 

 make signalling impossible over long lines with numerous instruments. Furthermore, 

 the continuous current that would flow through the transmitter circuit would quickly 

 exhaust a battery of the open-circuit type such as is used in the telephone. Accord- 

 ingly, in all telephones a device is installed by which the talking circuit is cut off from 

 the line when not in use and the transmitter circuit is opened. This is accomplished 

 by the switch-hook which is so arranged that when the receiver, which ordinarily is 

 suspended from it, is removed, the talking circuit is automatically thrown into the 

 line. Conversely, when the receiver is placed back on the hook, the depressing of this 

 hook opens the circuit. A large number of devices for accomplishing this have been 

 invented. The one shown in Fig. 80 is the type employed in the Forestry Branch 

 standard instruments in wall sets and desk sets. In hand sets such as the 1375-A or 

 the 1004-A portable instruments, push-button devices are. used to accomplish the same 

 result. It is well, perhaps, to note that in case one of these push-buttons is depressed 

 for any long period, the small battery of the portable instrument is very likely to 

 become exhausted. This sometimes happens with the 1375-A hand set where the 

 push-button projects proi linently from the side of the hand set and when packed for 



