THE CARBON MICROPHONE 



171 



voltage across the contact or microphone is large. These noises are 

 irregular like frying or crackling. Also, if a contact be viewed under 

 a microscope, bright spots are sometimes seen. These facts have led 

 many to think that small arcs are always present and are responsible 

 for microphonic action. Hughes was very much inclined to this view. 



Fig. 10 — Cross-section of the barrier type transmitter used in modern handset 



instruments. 



There were reasons for supposing that the heating of the contact is 

 a necessary factor in microphonic action. This point of view was 

 supported by Preece, who wrote in 1893, "Indeed there are many 

 phenomena such as hissing and humming that are clearly due to 

 what is known as the Trevelyan efifect, that is, the motion set up by 

 expansion and contraction of bodies which are subjected to variation in 

 temperature. This at least tends to favor the heat hypothesis as 

 does also the fact that with continuous use some transmitters become 

 essentially warm." 



