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BELL SYSTEM TECHNICAL JOURNAL 



of the speech current. In order to avoid distortion, severe require- 

 ments were imposed on the straightness of the edges of the tape, and 

 on the strength, in order to permit stretching to give a natural period in 

 excess of 7,000 cycles per second. To obtain these properties special 

 heat treatments and methods of rolling the material had to be de- 

 veloped. 



Fig. 13 — Light valve used in recording of sound pictures. 

 AA'^ — Wind-lasses. 



B — Pulley supported by spring. 

 CC^ — Insulating bridges. 



Ferrous Metals 



Some interesting problems have been encountered in the use of 

 ferrous metals in telephone apparatus, particularly in the operator's 

 calling dial. Considerable trouble was encountered from slippage of 

 the dial governor resulting from premature wear or breakage of the 

 tips of the pawls or the teeth of the pinion. These parts had been made 

 out of low carbon steel which had been found satisfactory for the 

 subscriber's dial. The operator's dial, however, being used for a 

 greater number of times, presented a more severe condition and case 

 hardening was applied to obtain better wear resisting properties. 

 This treatment was found to be unsatisfactory because the parts have 

 thin sections and the combined weight of the two parts amounts 

 to only 2 grams. Case hardening either produced too deep a case 

 giving brittleness or too shallow a case which soon wore through. 

 A nickel-chrome steel, originall}- developed for the automotive industry 

 was finally adopted for the pawl and pinion combined with a special 

 heat treatment. It was thus possible to obtain a useful life of 8 million 

 operations as compared with an average of }4 million operations for 

 the steel formerly used. This is another instance in which an increase 



