Abstracts of Technical Articles from Bell System Sources (01 January 1931)

Jour. Acous. Soc. Amer., July, 1930. Jour. Acous. Soc. Amer., July, 1930. Science, Nov. 7, 1930. 149 150 BELL SYSTEM TECHNICAL JOURNAL contact behavior of granular carbon of the type used in commercial microphones. A technique is described whereby contacts--either singly or in groups--may be studied under contact forces of the order of 1 dyne. Through a study of the temperature coefficients of resistance of such contacts it is possible to conclude that the conducting portions of the contact junctions are of the nature of carbon and that new contact points are established or broken when the resistance is varied in a reversible resistance force cycle. 

The experiments show that for such reversible cycles the relation between the resistance and force is of the approximate form R = K(F)~n. The exponent n varies considerably from cycle to cycle but its average value depends on the force limits. The largest values of n are obtained with the aggregates of granules under such conditions of force limits that the elastic strains must be relatively large. A maximum mean value substantially independent of the force limits over a wide range closely approximates the value 7/9. This value 7/9 is the maximum given by a theory of contact resistance worked out by F. Gray, assuming that the contact is made between two spheres of conducting material having surface roughness equivalent to an assembly of minute spherical hills. On account of the elasticity of the material both the microscopic area of contact between the spheres and the microscopic areas of contact between the hills increase with contact force.