78 BELL SYSTEM TECHNICAL JOURNAL 



the indistorted pressure that can be developed by the set is Hmited in 

 magnitude. Also the amplification is a function of the variation of the 

 power supply voltages. Hence it is necessary that these characteristics 

 be measured. As a;n illustration of the different factors involved, charac- 

 teristic curves for a recently developed vacuum tube aid are shown. 



Some Mechanical Aspects of Telephone Apparatus. Part 1 — Development 

 of Crossbar Switch as a Typical Unit of Automatic Dial Telephone System J 

 J. D. Tebo and H. G. Mehlhouse. One half the telephones in the world 

 are in the United States. A total of 100,000,000 calls a day are made on 

 this equipment, on matters large and small — routine or emergency. Espe- 

 cially in these days of national emergency, communication facilities have 

 become indispensable to our defense organizations, vitally assisting in 

 speeding up the program of national safety. 



Americans have grown to accept their telephones as a necessity, seldom 

 realizing the vastness of the system and the spirit of service in the large 

 organization constantly striving to maintain and improve a means of 

 communication already unexcelled. Seldom is it realized that the equip- 

 ment in the home or office is only a very small part of the facilities required 

 in this business of telephoning. A network of wire literally covering the 

 nation from coast to coast, and intricate switching devices for interconnect- 

 ing the telephones throughout the nation and most of the world all combine 

 to transmit the spoken word quickly and faithfully. 



The vastness and complexity of such a service must of necessity require 

 a wide range of equipment which, in the Bell System, totals 44,000 kinds of 

 apparatus involving 170,000 different parts. Some of the mechanical 

 aspects of such equipment are presented in this paper, particularly those 

 having to do with design and manufacture, jointly handled for the Bell 

 System by the Bell Telephone Laboratories and Western Electric Company. 

 Obviously, the entire range of telephone equipment cannot be included. 

 This paper is therefore intended to deal with a specific telephone switch- 

 ing mechanism, known as the crossbar switch, which is used in the latest 

 dial telephone system. 



' Mech. Engg., May 1942. 



