140 BELL SYSTEM TECHNICAL JOURNAL 



in which standard frequencies are derived from a 5-megacycle oscillator by 

 a series of frequency dividers. The advantage of obtaining standard fre- 

 quencies by frequency division rather than by frequency multiplication is 

 pointed out and the characteristics of the regenerative frequency dividers 

 used are discussed. 



Some Mechanical Aspects of Telephone Apparatus}'^ J. D. Tebo and H. 

 G. Mehlhouse. Part I. It is seldom reahzed that the vastness of the 

 Bell System requires such an enormous amount of equipment and wires for 

 handling the 100,000,000 calls per day made by the people in the United 

 States. A total of 44,000 kinds of apparatus involving 170,000 different 

 parts are required. The crossbar switch, described in this article, is the 

 specific telephone switching mechanism used in the latest dial telephone 

 system. 



Of particular interest in the crossbar switch is the design of contact springs. 

 These springs are essentially thin, metal beams of a rectangular cross section 

 but of varying cross sectional area along their length. They are clamped 

 at one end and are subject to bending as compound cantilevers. Since 

 the clamping is necessarily between relatively soft insulating materials, 

 the determination of the effective length of the spring in determining its 

 true deflection curve required the use of unique methods. Likewise, since 

 the springs are punched out of sheet stock at an angle to the grain direction 

 of the material, the modulus of elasticity does not remain the same for equal 

 cross sections of the same material. Consideration of these points was 

 necessary in determining the strength of the magnets for operating the con- 

 tact springs, as well as to insure that the stresses introduced in the springs 

 would not be excessive. 



To study the motions of the various parts of the switch, both high speed 

 motion pictures and the "rapid record" oscillograph were used. The oscil- 

 lograph was provided with means for obtaining" shadowgrams" of the actual 

 movement of parts simultaneously with the changes in the electrical charac- 

 teristics of the magnets and contacts. The use of both the camera and 

 oscillograph provided valuable data for making improvements, both in 

 design and operating characteristics. 



Part II. The manufacture of crossbar apparatus is accomplished on a 

 product basis; that is, the entire range of manufacturing operations is segre- 

 gated into one division, and practically all operations from raw material 

 to the completed product are performed in this division. More than 150 

 kinds of parts totaling an annual demand of 200,000,000 individual pieces 

 are required to produce the crossbar switches. 



To produce these parts requires a number of special machines, tools, and 



11 Mech. Engg., May 1942 and June 1942. 



