.irPl.lCATIOS OF ST.lllSTIC.il. .Ml-.TIIOPS 49 



thenisflvi-s represent the current through the transmitter and, there- 

 fore, are in\ersely proportional to the resistance. All five curves 

 were ohtainoil with the s.inie carbon in the same chamber by varying 

 merely the configuration of the granules by slightly tapping the carbon 

 chamber. 



All of these effects can be modified to a large extent by varying 

 the process of manufacture of the granular material. In practice it 

 is necessary to know why slight changes in the manufacturing process 

 cause large variations in the resistance characteristics of the carbon. 

 The same process that improves one microphonic property may prove 

 a detriment to another. It is in the solution of some of these problems 

 that statistical methods have been founfl to be of great value in the 

 interpretation of the results. 



Whereas the physicist ordinarily works in the laboratory under 

 controlled contlitions, the engineer must work under commercial 

 conditions where it is often impractical to secure the same degree of 

 control. More than 1,500,000 transmitters are manufactured every 

 year by the Bell System. Causes of variation other than those intro- 

 duced by the carbon help to control the transmitter. For example, 

 variations may be introduced by the process of assembly, or by 

 differences in the piece parts of the assembled instrument. The 

 measure of the faithfulness and efficiency of reproduction depends 

 fundamentally upon the human ear. Obviously all transmitters 

 cannot be tested. Instead, we must choose a number of instruments 

 and from obsers^ations made on these determine whether or not there 

 is any trend in the manufactured product. Xaturally we may expect 

 to find certain variations in the results according to the rules of chance. 

 To take the simplest illustration, we may flip a coin 6 times. Even 

 if it is symmetrical we may expect occasionally to find all heads and 

 occasionally all tails, although the most probable combination is 

 that of 3 heads and .3 tails. We must, therefore, determine first of all 

 whether or not the observed variations are consistent with those due 

 to sampling according to the laws of chance. If there is an apparent 

 trend in pnxluct, the data should be analyzed in order to determine, 

 if possible, whether it is due to lack of control in the manufacture 

 of carbon or to some other set of causes such as mentioned above. 

 Because of economic reasons we must keep the number of observations 

 at a minimum consistent with a satisfactory control of the product. 

 Here again it has been found that the application of statistical methods 

 is necessary to the solution of the problems involvcnJ. 



Before considering the problem of the measurement of efficiency 

 and quality of the transmitter, let us consider the schematic diagram 



