TELEPHONES 451 



The party-line system in most common use is what is known 

 as the bridging-bell system, so called because all bells are 

 connected to the line in multiple and thus form bridges over 

 which current can pass from one side of the line to the other. 



Bells of relatively high resistance, 1,000 to 2,500 ohms, 

 must be used for bridging party lines, since if the bells are 

 not of high resistance a large portion of the voice current 

 will pass through them, and poor transmission will result. 

 Furthermore, uniform signaling cannot be obtained without 

 the use of such resistance bells, because if low-resistance bells 

 were used on bridging circuits the one nearest to the calling 

 station would consume an undue portion of the signaling 

 current, and those at a distance would ring feebly or not at all. 



For calling the various stations systems of signals con- 

 sisting of varying numbers of rings or of combinations of 

 short and long rings are used on party lines such as just 

 described. This system of signaling is known as the code- 

 ringing system. There is no very definite limit to the num- 

 ber of stations which can be connected to one line, but it 

 must be kept in mind that all of the bells respond to every 

 signal, and that, in addition to the annoyance to the persons 

 who must hear all of the signals, whether for their station 

 or not, many mistakes are made where the stations and 

 signals are numerous. The number of stations connected 

 to one line should, therefore, be restricted. For city exchange 

 service the number is usually limited to four, and on rural 

 party lines double this number would be a good limit. 

 Rural lines have been reported with thirty connected stations. 



A battery is good for only a limited number of talking 

 minutes. With local battery instruments, it should be 

 remembered that the battery circuit is closed while one is 

 "listening in" as well as when one is talking. Furthermore, 

 entering the circuit offers a leak for the voice currents of 

 those talking. 



Selective signals. There are party-line systems known as 

 selective systems, wherein any station may be called without 



