45° ENGINEERING ON THE FARM 



considering each case separately, while many offer local 

 battery connections only. Still another arrangement pro- 

 vides central battery for signaling the central office, and 

 local battery for all talking. 



Where central battery is provided, either for signaling the 

 central office or for this purpose and talking as well, a desir- 

 able feature is obtained in that calls for the central office 

 from any station on the line do not operate other bells if 

 there are other stations connected to the line. On the 

 other hand, one station cannot call another on the same line 

 except by signaling the central office and having the desired 

 second station on the same line signaled from that point. 

 Where local battery is employed, it is possible for any sta- 

 tion to call any other on the same line, except where spe- 

 cially constructed telephones are used for the purpose of 

 insuring that all calls shall be supervised by the central 

 office. Such telephones have direct-current generators, and 

 their bells, being responsive to alternating current only, do 

 not operate when the generator is turned, but the signal at 

 the central office does, as it is designed to respond to direct 

 current. 



Still another type of local battery instrument, which is 

 sometimes used on metallic circuits, is one provided with a 

 key or switch by means of which the generator, which is 

 of the alternating-current type, can be connected either across 

 the line, when it will operate the bells on the line, or between 

 the wires of the line and ground, when it will operate the 

 central-office signal, leaving the bells at the various stations 

 silent. It will be seen, therefore, that the exact type of 

 instrument selected must depend upon the facilities contained 

 in the switchboard at the central office for terminating cir- 

 cuits of the kind under consideration. 



Party lines. Regarding the second point, that of party- 

 line service, the telephone company should be consulted, and 

 its facilities should be taken into account in the selection of 

 instruments (telephones) and the construction of the line. 



