232 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



analytical and quantitative approach is demonstrated by the much higher 

 electrical output than that of the 1927 transmitter. This is shown by the 

 comparison of Curves B and C of Fig. 4(a). 



For the 1937 handset, the objective was set of making the receiver also 

 free of resonance. The method of obtaining this result in terms of the equiv- 

 alent electrical circuit is discussed in the W. C. Jones paper^^ of 1938, which 

 shows also the electrical analog for the transmitter of this handset. A com- 

 parison of the receiver of the 1927 and 1937 handsets is given by Curves 

 B and C of Fig. 4(b). It is seen that the diaphragm resonance is completely 

 eliminated. The Jones paper indicates how the air spaces associated with 

 the diaphragm and an acoustic resistance element are employed to control 

 the motion of the diaphragm. 



In the 1937 receiver, three special magnetic alloys are employed — per- 

 mendur for the diaphragm, 45 percent permalloy for the pole pieces and 

 remalloy for the magnets. In the manufacture of these receivers, each one 

 is magnetized to its optimum value. ^° 



In the instruments of the telephone set of 1950, the appUcation of these 

 design procedures has been carried still further. As brought out in a cur- 

 rently published paper,^^ the performance requirements for these instruments 

 were set on the basis of what it was desired to have in the way of bandwidth, 

 frequency characteristics, and efficiency. The instruments were then devel- 

 oped to attain these characteristics. 



The response of the 1950 transmitter is shown by Curve D in Fig. 4(a). 

 The shape of this response was deliberately planned to be as shown in order 

 to approach the characteristic of the air transmission path. The gain over 

 Curve C of the 1934 transmitter is obtained with a decreased size of dia- 

 phragm and unit. 



Also in the 1950 transmitter, a further improvement has been made in 

 the granular carbon to increase its stabihty with time. For many years, in- 

 tensive studies^^' ^ have been made of the performance of granular carbon 

 in the telephone transmitter to understand the contact action and to deter- 

 mine the causes of aging with use and time, and the means of alleviating 

 these effects. From these and other studies of the structure of the chamber 

 containing the carbon, have come remarkable results in improving the per- 

 formance of this very critical mass of loose granules. It has been stated that 

 the telephone system is built around a loose contact which is a thing that 

 the electrical engineer hopes to avoid. The fact is that, today as a result of 

 all this work which has been done on the use of granular carbon in the trans- 

 mitter, Uttle of value would be gained in the quality of reproduction in 

 commercial telephony by the replacement of the current desgins of this 

 simple low-cost means of making the conversion between acoustic and elec- 

 trical energy, by a combination of a passive device with a vacuum tube form 



