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BELL SYSTEM TECHNICAL JOURNAL 



stimulated others to work on the problem of a variable resistance 

 element and many new devices appeared in the next few years, the 

 most sensitive of which utilized a single loose contact, carbon in one 

 form or another being used as the contact material. 



Fig. 3- 



-Berliner's first single contact microphone, invented in 1877, employing a 

 metal-to-metal contact. 



Figure 3 shows Berliner's first successful model consisting essentially 

 of a metal contact pressed against a metal diaphragm. This was 

 developed later into a carbon-to-carbon contact along the same 

 lines (Fig. 4). 



Hughes, too, used metal in his first successful attempt at trans- 

 mitting sounds. Only three ordinary nails were required to demon- 

 strate the great sensitivity of loose contacts to acoustical vibrations 

 (Fig. 5). Hughes later developed the pencil type of microphone 

 (Fig. 6) in which carbon was used. It was the forerunner of many 

 practical devices developed along this line. 



More rugged, reliable and permanent than either of these types was 

 the Blake transmitter shown in Fig. 7. It utilized a metal-to-carbon 

 contact and it owed its success to the mechanical control of the 

 contact pressure. This instrument was used for many years by the 

 Bell System. 



Then came the Hunnings or the first of the granular carbon micro- 



