40 BELL SVSrnM TECIIXIC.IL JOURNAL 



imposed on telephone relays there is not the wide range of operat- 

 ing conditions to be met under which most telegraph relays are re- 

 quired to operate. The numbers in\ol\ed are usually much less so 

 that economies in production play a somewhat less important role 

 and the cost is not quite such an important item. Similarly the 

 methods of assembly and mounting afford a somewhat wider latitude 

 than can be permitted where many thousands of relays must be 

 mounted in comparatively small space. 



Because of the exacting requirements imposed on telegraph relays 

 and to insure continuity of ser\ice as far as possible, they are usuall>' 

 made interchangeable to a much greater degree than telephone relays. 

 They may be connected by means of screws instead of soldered con 

 nections or they may be inserted in the circuit by means of spring 

 clips in a connecting block. 



In a telegraph system speed of operation and reliability are the 

 most important requirements and are very large factors in determin- 

 ing the mechanical design of the relays. The relay must operate 

 quickly and accurately so as to cause as little distortion as possible 

 to the signals. In addition it must be extremely rugged and main- 

 tain its adjustment well throughout long continued operation. A 

 very ordinary day's work for a telegraph relay requires the reliable 

 operation of its contacts several hundred thousand times and it may 

 be called upon to open and close its contacts a million times a da\'. 

 Where a telephone relay might hesitate and still pull up and perform 

 its function proi)erl\' or might make uncertain contact at first, such 

 behavior on the i)arl of a telegraph relay would result in false 

 impulses and woiilii (|iiirkl\' call for a readjustment or a change ol 

 relays. 



With the exception of some of the alternating current signaling 

 relays in telephone circuits the energy available in telegraph relays 

 is usually less than that available for telephone relays. The more 

 sensitive relays are called upon to operate from line current which 

 has been attenuated by leakage or by parallel paths and which may 

 have been limited at the distant station. Systems operating over 

 open wire lines arc usually restricted to about .075 ampere at the 

 sending end and in cable the normal current is about .005 ampere. 

 This difference is not as great in actual operation as would appear 

 since the open wire system operates on a ground to ground basis and 

 the cable on a metallic basis. In operating from ground ai one 

 station to ground at another differences in ground potential and 

 leakages occur which re(|uire a greater margin than is ni'Ci'ss.iry with 

 the metallic system. 



