266 BELL SYSTEM TECHNICAL JOURNAL 



Materials 



In the practical design of modern telephone instruments we owe a 

 large debt to the chemist and the metallurgist. Modern molding 

 materials and processes are utilized in order to secure forms of ap- 

 paratus satisfactory from the standpoint of appearance and of me- 

 chanical strength. The newer types of permanent magnet steel, to 

 the development of which your countrymen have contributed so 

 largely, provide possibilities of light-weight and very efficient magnetic 

 structures. 



It is a most striking circumstance that commercial telephony is 

 dependent upon the performance of a small mass of carbon granules 

 in the transmitter. No single material entering into the construction 

 of telephone apparatus has therefore greater importance. In America 

 at least, transmitter carbons are largely derived from a certain specially 

 selected anthracite coal. In its natural state, this coal exhibits none 

 of the characteristics required for its use in a transmitter. These 

 characteristics or properties are secured by heat treatment. These 

 heat-treatment processes were for many years the result of empirical 

 development and were not well understood or, as we now recognize, 

 adequately controlled. This resulted in a product of uncertain quality. 

 An important task of the Laboratories was therefore to study each 

 step in the process of producing carbon and to develop a process 

 definitely specified at each step, which would be capable of giving the 

 desired uniform quality. The results so far obtained have had very 

 important reactions upon transmitter performance. The Laboratories 

 have also set themselves the more elementary task of understanding 

 the fundamental properties of carbon contacts. One important element 

 of this research is to determine the causes of resistance changes 

 produced when the compressive force on a mass of carbon granules is 

 changed. It is too early to report results from this research, but it seems 

 clear that granular carbon will be an important element in the design of 

 transmitters for many years to come, and we should seek to obtain 

 complete fundamental knowledge of its operation. 



Testing Methods 



Broadly speaking, methods of testing have been developed, first, to 

 enable the development and design engineer to determine quantita- 

 tively the various performance factors of the apparatus under develop- 

 ment, and second, to determine how well the apparatus which has 

 been developed performs under service conditions. In the Labora- 

 tories, we have over the last twenty years developed methods for 

 measuring the physical constants of the apparatus involved so that 



