THE MAGNETO TELEPHONE 137 



The standard Forestry Branch repeating coil is a special type adapted so that 

 it may be used for several purposes. In this coil there are four independent wind- 

 ings, each with a pair of outside terminals. By joining two pairs of these windings 

 together by jumpers 1 between the proper terminals, as shown in Fig. 56, a repeating 

 coil of two independent windings is secured, which may be used for the purpose 

 above described. This arrangement of coils, however, also makes possible the making 

 of a connection to the exact centre of either one of the two windings, since each is 

 composed of two separate but similar windings joined by an external jumper. Such 

 a connection to the centre of one of the windings of the repeating coil is shown in 

 Fig. 56 by the heavy line leading from the terminal marked 3 and labelled " To 

 Phantom." The coil, therefore, may also be used to secure two distinct telephone 

 circuits from a single metallic line as shown in Fig. 87. In this figure the circuit 

 A B is metallic with a repeating coil at each end. Transmission between the through 

 stations A and B over the metallic line takes place inductively through the two coils. 

 Each coil consists of two windings. Winding 1 is similar to 4; winding 2, to 3. 

 The second circuit C D is taken off from the centre of winding 2 and winding 3, 

 thus assuring that the resistance in the coil on both sides of the point of connection 

 is the same at each end. To work perfectly the ohmic resistance of the two line 

 wires must also be equal. It will then be apparent that current entering at 2 from 

 station C will divide evenly at 2 and pass over both wires to the coil at station B, 

 where it will again unite and pass through D to the ground. At any intermediate 

 station such as E, for instance, no effect will be noticeable, because since both wires 

 carry an equal current with no difference of potential there is no tendency for cur- 

 rent to pass from one to the other and therefore it cannot flow through the apparatus 

 at station E. The circuit taken out of the centre of the coils at 2 and 3 is called a 

 " phantom " circuit and by its use two distinct independent circuits, one grounded, 

 the other metallic, may be secured from a single pair of wires. Similarly three dis- 

 tinct metallic circuits may be secured from two pairs of wires, and various other com- 

 binations whereby several independent telephone circuits may be superimposed on 

 the same wires are possible. 



Section 87 Methods of Connecting Telephones to Line 



Forest protection telephone systems are always party lines; that is, there are a 

 greater or less number of instruments all connected to the same line at different 

 stations between its terminals. There are two general systems employed for con- 

 necting up the instruments on a party line of this kind : the series system and the 

 bridging-bell system. Of these, only the bridging-bell system is of any use on forest 

 lines and, in fact, the series system is now little used for any purpose. 



1- SERIES SYSTEM 



This is the oldest method of connecting 1 .the several instruments on a party line, 

 but is now so little used and is so inapplicable to forest-protection telephone lines that 

 only the briefest consideration is desirable. The method of connection is fully illus- 

 trated in Figs. 88 and 89, the first of which shows a series connection on a metallic 

 circuit and the second a series connection 011 a grounded circuit. It will be easily 

 seen from these diagrams that the talking current between any two telephones on the 

 line has to traverse all the bell coils of the intermediate telephones 1 . These, there- 

 fore, must be wound to very low resistance, generally about 80 ohms, and even then 

 the current is so weakened by the combined effects of resistance, impedance, induc- 

 tion, and leakage that speech transmission i? 1 impracticable except over a short line 

 with relalively few telephones connected to it. 



