THE CARBON MICROPHONE 



179 



This relation has a theoretical basis in the Joule heating of the 

 contacts due to the passage of current and contains the assumption 

 of a value of Wiedemann Franz ratios characteristic of solid carbon.* 



IXIO' 



1X10- 



1X10 



• BY CHANGING TEMPERATURE 



o CALCULATED FROM T = T^ + 40 V 

 V 



0.001 



0.01 0.1 



VOLTS 



1.0 10 



50 100 



TEMPERATURE IN°C 



Fig. 15 — Characteristics showing the effect of voltage and temperature on contact 



resistance. 



These curves have substantially the same slope as A, which is a 

 characteristic measured by heating a contact in the furnace, the con- 

 tact voltage being sufficiently small to avoid appreciable heating of 

 the contact due to this cause, and also with B, which was obtained with 

 a solid carbon wire produced in a manner to simulate closely micro- 

 phone carbon. We are able to conclude from measurements such as 

 these that the nature of the conducting portions of contacts is that of 

 solid carbon both for air and vacuum and that the departures from 

 Ohm's law — at least up to 1 volt — are due to the Joule heating of the 

 contacts. 



From measurements similar to these in which we show that the 

 admission of air has no effect on the temperature coefficient of re- 

 sistance — although it produces a marked increase in the resistance at 

 any particular temperature — we are also able to conclude that the 

 presence of adsorbed air does not alter the nature of the conducting 

 portions of the contacts but merely limits their areas. 



* This theory, based on earlier work of Kohlrausch, was worked out in useful 

 form independently in Bell Telephone Laboratories (unpublished work) and by R. 



Holm {Zeit. Tech. Phys., 3, 1922). It gives the approximate relation, const, j^ . , 



as the increase in temperature above room temperature, V being the contact volt- 

 age, and Ko/ffo the Wiedemann Franz ratio for the contact material. 



