McClklland and Dowling — Electrical Conductivity of Powders. 57 



a freshly formed layer not exposed to any transverse field. The curve is 

 expressed very accurately by the equation 



C=aV+bV\ 

 where is the current and V the electromotive force. Curve B is obtained 

 by plotting the ratio C/V against V, and is very accurately a straight line. 



Fig. 6 refers to a layer the conductivity of which had been very greatly 

 increased by the continued application and removal of a large transverse 

 field ; in fact, the transverse field was continuously applied and removed 

 during the observations. For the greater part of the range the curve is a 

 straight line, and may be expressed in the form 



G = Jc(V-v). 



The current is proportional to the excess of the E.M.F. above a certain value. 



The above curves figs. 5 and 6 are typical of the results obtained for all 

 the layers we have tested, but different layers show differences of detail 

 which we are not now entering upon. 



We have tried how the conductivity is affected by allowing a current to 

 flow through the layer ; in particular we have tried whether the conductivity 

 of a layer which has been exposed to a transverse field would fall off any 

 more rapidly if a current were kept flowing between the tinfoil strips. 

 When there is a large E.M.F.- between the tinfoil strips, small sparks form 



