170 BELL SYSTEM TECHNICAL JOURNAL 



associated with the action that it is hard to determine which of them 

 is essential. It is therefore not surprising that there was great diver- 

 sity of opinion amongst the early inventors. 



Fig. 9 — The solid back transmitter invented by White in 1890. 



For instance, experiment shows that contacts tend to move apart 

 when in the act of transmitting sound. This led many, amongst 

 them Berliner, to hold the view that an air film is necessary for micro- 

 phonic action, that the current somehow passes through the film, and 

 that the variation of the current is due to the variation of the thickness 

 of the film. This view, however, was partly discredited by experi- 

 ments showing that the moving apart was probably due to a heating 

 of the contact through the passage of current and hence that it is 

 not a necessary accompaniment of microphonic action. 



Again, when one listens through a receiver placed in a circuit con- 

 taining a "loose contact," noises are heard, especially when the 



