RELAYS IM THE BELL SYSTEM 3 



iiivolvod in a local connection may be 14(). Wluii loll ((imiections 

 are in\olved even greater dependence is placx-d dm relays to render 

 service. A New York-San Francisco connection recjuires over 200 

 relays and very frequently connections are established which require 

 more than 300 relays. The majority of these relays are normally 

 available for doing their bit to provide telephone service to any one 

 of a large number of subscribers. As a matter of fact, approximately 

 'JO per cent of the millions of relays in the Bell System today are 

 available for and ma\- be called iipna to serve any subscrilier or user 

 of the telephone. 



A t>-pical manual office ser\ing 10, 000 lines would ha\ e from 40, ()()() 

 to 6-T,000 relay's and their total combined pull if apjilied at one point 

 would be sufficient to lift ten tons. In the larger machine switch- 

 ing offices there may be as many as 140,000 relays which require in 

 some instances power plants capable of handling peak loads of 4,000 

 amperes at 48 volts. 



Referring to Fig. 1, the sjjaee re(|uired for mounting some of the 

 relays in an office will be seen. This is a picture taken in one of the 

 New York offices which has o\er 60,000 relays and the racks shown 

 contain about 22,000 of these. The covers have been removed 

 from a number of the relays in the foreground. Instead of group- 

 ing the relays compactly as in a manual office it is the practice in 

 machine switching offices to mount them in close association with 

 the related apparatus units. This is illustrated by the photograph 

 of sender circuit relays shown in Fig. 2. 



IXVENTIO.\ OF THE ELECTROMAGNET 



Prior to 1820, the electro-magnetic structure, now known as a 

 relay, was an impossibility because the scientific facts on which it 

 is based had not been discovered. In the winter of that year. Oersted 

 of Copenhagen established that a mechanical eflfect could be producetl 

 on a magnetized needle by a current of electricity. Oersted dis- 

 covered that a magnetic needle would be deflected from its normal 

 position when held parallel to a wire conveying an electrical current 

 and that the deflection would be to the right or left, according to the 

 direction of current flow. This discovery aroused such interest among 

 scientists and philosophers that the best minds in Europe were 

 engaged in speculation and experiment, so that further discoveries 

 of great importance followed rapidly. Arago in Paris and Davy in 

 London, working independently, soon observed that, if an electric 

 current passed through a wire of copper or any other material, the 

 wire had the power of inducing permanent magnetism in steel needles. 



