TELEPHONES 453 



the use of this system be restricted to lines terminating in a 

 central office, where the necessary machinery for producing 

 the required current may be maintained, and that all stations 

 be rung from the central office. 



The polarity system involves the use of bells so designed 

 and constructed that they will respond to pulsating or inter- 

 rupted currents of one polarity only. It is in quite general 

 use for four-party selective service on metallic circuits, a 

 bell responsive to each polarity being connected between 

 each side of the line and the earth. This system, like the 

 harmonic system, is practically confined to lines terminating 

 in central offices, owing to the somewhat involved nature of 

 the requirements for signaling currents. It is also necessary 

 with this system to have the central office call all stations 

 on the lines. 



In some instances semi-selective service is rendered by 

 means of two stations connected in the same electrical rela- 

 tion to the line as each one in the systems just described. 

 The users of one of the two identically connected stations 

 are then directed to" answer one ring and to disregard two 

 rings, while the users of the other are directed to respond 

 to two rings and to disregard one ring. This method is 

 resorted to where it is desired to accommodate more sta- 

 tions on a line than can be handled in the strictly selective 

 manner, and still to derive some of the benefits of this 

 system. 



Line circuits. The question of the type of circuit to con- 

 struct, whether ground or metallic, does not depend so much 

 on the type of apparatus in use in the central office as on the 

 energy supply and the type of party-line service. A metallic 

 circuit is to be recommended in all cases, owing to its freedom 

 from cross talk and other forms of inductive as well as 

 atmospheric disturbances. Even though the switchboard 

 on which it is desired to terminate a privately owned line is 

 not constructed for handling metallic circuit lines, it would 

 be possible to provide apparatus at the central office for 



