TflE MAGXETO TELEPHONE 



131 



transportation with the receiver cord wound around it, it sometimes is depressed for a 

 long time. Most of the trouble with exhausted batteries in this set is probably due to 

 failure to guard against this accident. 



8 ACCESSORY EQUIPMENT 



In addition to the essential parts of the telephone previously described there are 

 several pieces of apparatus generally used on forest lines about which it is desirable 

 that some information be available to those in charge of such lines. 



These include the following : . 



(a) Condenser 



(b) Howler 



(c) Lightning-arrester or -protector 



(d) Switches 



(e) Repeating coils 



(a) Condensers. If two conductors, preferably in the form of thin plates such as 

 layers of tin-foil, are placed close together with a non-conducting material or dielectric 

 between them so that they are nowhere in direct contact and are then connected, one 

 to each pole of a voltaic cell, it will be found that a certain amount of current will 



ROLLED CONDENSER 



Me. 21- 

 CAPACITY 3L W. F. 



Fig. 81 Rolled condenser 



flow from the cell to the plates until they become fully charged with static electricity. 

 Such a device is called a condenser. The ability of an arrangement of this sort to 

 receive an electric charge, generally termed its capacity, varies in direct proportion 

 to the area of the plates, inversely as the square of the distance between the plates, and 

 directly as the specific inductive capacity of the dielectric. It is not essential that 

 the conductors be in the form of plates, however. Any two surfaces, such, for 

 instance, as the surfaces of two telephone wires placed parallel to each other and 

 insulated from each other as on a metallic pole line, will act in the same manner. So 

 also will a single wire line, the earth's surface forming one plate, the wire the other, 

 and the air between them acting as the dielectric. A great many of the difficulties 

 encountered in securing good telephonic transmission are caused by this electro- 

 static capacity of the line. This, however, is aside from the subject of condensers as 

 exemplified in the accessory equipment of the telephone. 



It was previously stated that if the talking circuits of the various instruments 

 on a line were left in the circuit continuously instead of being cut out by the switch- 

 hook when the receiver is hung up, it would be impossible to ring up the stations on 

 the line, owing to the leakage of the calling current through the receiver circuit. The 

 same thing takes place when on party lines a number of receivers are taken off the 

 iooks by persons " listening in." Signals could not be transmitted over such lines 



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