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)~"'. The exponent n varies considerably from cycle to cycle 

 but its average value depends on the force limits. The largest values 

 of w 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. A strained aggregate of granules may therefore be made 

 to behave like an ideal single contact between spheres having a rough 

 surface. 



For single contacts and for aggregates at small strains the value of n 

 falls below the minimum value 1/3 which is accounted for by theory. 

 This is associated with internal contact forces, or cohesion, which render 

 the contacts relatively insensitive to changes in the applied force. 

 The existence of cohesion is readily demonstrated by the fact that 

 contacts always require a finite force to break them even when no 

 current has passed through the contact. 



The Architecture of Living Cells — Recent Advances in Methods of 

 Biological Research — Optical Sectioning with the Ultra- Violet Micro- 

 scope} F. F. Lucas. In previous papers of the past few years the 

 development and application of the ultra-violet microscope to the 

 science of metallography have been described. 



Metallography, at first thought, appears wholly unrelated to his- 

 tology or other branches of biology but the two branches of science do 



* Proc. Nat. Acad, of Sciences, Sept., 1930. 



