612 BELL SYSTEM TECHNICAL JOURNAL 



have a higher electrical conductivity than their alloys. The problem 

 of conductivity of copper is therefore essentially one of purity, and 

 has been fairly satisfactorily solved by the copper-refining industry. 

 We are, however, attacking some special problems in conductivity 

 of copper. 



Carbon 



A unique conducting material in the field of telephony is carbon. 

 A small mass of granules of this material in every transmitter serves 

 the all-important purpose of converting the variations of the mechani- 

 cal energy of the voice into equivalent variations of the transmissible 

 electric current. This the carbon does by variation of its electrical 

 resistance with variation of mechanical pressure. 



No other material approaches carbon in its usefulness for this pur- 

 pose, but there are still many obscurities about the functioning of 

 a carbon transmitter. Decades of physical and chemical research 

 have, however, established certain points. Transmitter carbons in 

 general are not highly active carbons in the sense in which we have be- 

 come familiar with that term in connection with absorptive charcoals. 

 Gas films on the carbon, however, do play some role in their micro- 

 phonic functioning. Of more practical importance for the present is 

 the evidence that the microphonic effectiveness of carbon is very much 

 dependent on the method of its preparation, and particularly the time, 

 temperature, and atmosphere in which it is roasted. Carbon for trans- 

 mitters is made from anthracite coal of maximum hardness and low ash 

 content. The roasting processes are designed to produce a material 

 of as high uniformity as possible, especially with reference to the hard- 

 ness, compactness, and abrasion resistance of the surfaces of the finished 

 product. It has come to be recognized that such physical character- 

 istics of transmitter carbon as these to a great extent determine, not 

 only its original effectiveness, but also its resistance to atmospheric 

 disturbances and its durability under the mechanical and electrical 

 forces exerted upon it during use. 



The hydrogen content of carbon has been found a useful index of the 

 time-temperature cycle to which it has been subjected during roasting. 

 As anthracite coal is roasted there is a progressive loss of hydrogen, 

 which, however, does not reach zero value until a temperature above 

 1500° C. is obtained, at which point the material is converted rapidly 

 to graphite. It is only at upper intermediate temperatures that satis- 

 factory transmitter carbons can be produced. 



The study of the hydrogen contents of coal and carbons and of re- 

 lated microphonic behavior has led to a definite theory that there are, 

 contrary to belief of some authorities, only two allotropic forms of car- 



