238 BELL SYSTEM TECHNICAL JOURNAL 



on these problems. Thus it has come about in the Bell System, as with 

 other large consumers of materials, that the investigation of materials 

 has been organized as a distinct branch of research and engineering 

 activity. Studies of the chemical, physical and metallurgical proper- 

 ties of materials are embraced in this work. In general the materials 

 engineer should not only be well versed in materials, but should also 

 have a good knowledge of the operating characteristics of the apparatus 

 to be designed. Thus he can discuss the materials side of the problem 

 with the designing engineer on equal terms and make his contribution 

 to the best advantage. The importance of a thorough knowledge 

 of materials in the telephone business will be appreciated from the fact 

 that, during 1929, it is estimated that about $590,000,000 will be spent 

 for additions to the Bell System plant. 



In telephony the general introduction of the dial system has imposed 

 more severe requirements than heretofore because of the need for the 

 utmost in reliability of performance of the large number of switches, 

 relays, etc., which are required to operate automatically with a mini- 

 mum of maintenance. In the central ofihce small size of apparatus con- 

 stitutes a very important consideration, not only because of building 

 space required, but the mass and travel of the automatic switches have 

 an important effect on the speed with which connections can be es- 

 tablished and hence on economy of operations. Thus, close control 

 of the quality of materials and the need for small, compact apparatus 

 are important design considerations. 



In a brief survey of progress in the development of materials, it will 

 be necessary to select a few typical items of interest. The items 

 selected deal primarily with the telephone problem as this is, at the 

 present time at least, the largest single factor in the communications 

 group. The subject may be divided broadly into insulating materials 

 and metallic materials. 



. Insulating Materials 

 Phenol Fiber 

 Considering first sheet insulating material, we have been using the 

 term "phenol fiber" to cover such materials as bakelite-dilecto, mi- 

 carta, formica and similar fibers made by various manufacturers. 

 Phenol fiber is used extensively in telephone apparatus. One of its 

 applications is in the sequence switch which has insulators alternating 

 with conducting segments, as shown in Fig. 1. The sequence switch, 

 which is used in the dial system, draws out an arc when in operation 

 which sometimes causes carbonization of the insulators. In some cases 

 a hole was burned through the insulator and in other cases the arc was 



