6 BELL SYSTEM TECHNICAL JOURNAL 



First Telegr.4ph Relays 



In 1824, Morse utilized the electro-magnetic phenomenon, revealed 

 by Sturgeon, and produced a telegraph system which was destined 

 to be the basis of all modern systems of communication. The at- 

 tenuation of the current from the sending to the receiving end of 

 the circuit had limited the satisfactory transmission of signals. 

 Morse overcame this difficulty by constructing an electro-magnet 

 which would repeat or "relay" the transmitted signals to another 

 circuit having an independent source of energy. The first electro- 

 magnet or relay designed by Morse was a cumbersome structure 

 weighing about 300 pounds, but it e.xerted a tremendous influence 

 on the art of communication as it served as a stimulus for the devel- 

 opment of the complex systems of the present day. When this 

 relay was redesigned its weight was reduced to 70 lbs., but as the 

 laws of electro-magnetism became more generally understood and 

 new materials became available, such great changes occurred that 

 the present telegraph relay weighs about 3 pounds, and one of the 

 modern telephone relays of latest design weighs but 3.6 ounces. 



The General Problem 



The needs of the present day telephone and telegraph system have 

 produced a multitude of devices but none of them is of greater im- 

 portance than the relay, as it affords the means whereby the engineer 

 may put ideas into practice. When the limitations of available 

 relays prevent the satisfactory solution of a problem, requirements 

 for new relays are outlined and their development is undertaken if 

 a survey indicates that the aihantages to be obtained warrant the 

 expense. 



This does not mean that compromises are not made in the matter 

 of using standard designs, for in some instances, it would not be 

 economical to design a new type. Frequently, a relay is reciuired 

 to meet certain conditions in the plant for which the demand will be 

 comparatively limited, and it is obviously uneconomical to spend 

 time and money developing a new type provided a standard struc- 

 ture can be adapted to meet the requirements with sufficient precision. 



Just as the art of defense in warfare has matched the art of offense, 

 the art of relay design has kept pace with the demands of the cir- 

 cuit engineer. Relays are now required to operate on direct, and 

 pulsating current, and also on alternating current throughout the 

 entire range of frequencies which are used in communication. There 

 are fast relays, slow relays, polarized, high impedance, low impedance 



