THE CARBON MICROPHONE 



169 



are compressed between two polished carbon electrodes which are 

 immersed in the granular mass in such a way that the particles have 

 more freedom of movement than in the Hunnings instrument. This 

 relieves excess pressure without undue packing. 



Fig. 8 — Commercial model of the early Hunnings transmitter in which granular 



material was first used. 



In Fig. 10 we have a cross-sectional view of a modern handset 

 transmitter. This instrument, which is designed to operate in a 

 wide variety of positions, follows the Runnings' type in that the 

 granular mass rests against the diaphragm but it differs from it in 

 that the diaphragm does not act as an electrode. Both electrodes, 

 separated by an insulating barrier, form part of the containing walls 

 of the cell holding the carbon. This is the type which has recently 

 been studied in detail and of which a two dimensional model is shown 

 in Fig. 26. 



The carbon used in these instruments is made by a heat treatment 

 of anthracite coal. The particles are about 0.01 inch in size and 

 when magnified they look just like lumps of coal taken from the 

 domestic pile (Fig. 11). 



Speculations of the Early Inventors 

 Part of the difficulty in elucidating the microphonic action of the 

 "loose contact" arises because so many effects can be observed or are 



