McCi-i'-LLAND and Dowling — Electrical Conductivity of Powders. 55 



but the rapid increase of conductivity commences at about 100 volts for the 

 layer with a thickness of paraffin of l - 05 rnnis., and at about 220 volts for 

 the layer with a thickness of 2 - 19 rnnis. It has been noticed that a layer 

 which had been exposed to a large transverse field, and had lost most of 

 its conductivity with time, was more sensitive when the field was again 

 applied. Small fields produced a greater effect, and the very rapid increase 

 of conductivity occurred at lower voltages than at the first application. 



Volts. 



too 



2.00 



300 



Fig. 4. 



Experiments were then made to test whether the great increase of 

 conductivity produced by the application of a strong electric field would 

 take place if the field were applied gradually in a number of small 

 successive steps. By means of a potential divider of 100 coils of 100 

 ohms each in series, an electromotive force of 200 volts was applied to a 

 layer in steps of 4 volts, and gradually removed in the same way. Prac- 

 tically no increase of conductivity took place. When the 200 volts were 

 applied at once and removed in the usual way, an enormous increase of 

 conductivity occurred. The sudden application or removal of the E.M.F. is 

 therefore essential in order to produce the increase of conductivity. 



6. In all the experiments described above the E.M.F. applied along the 

 layer between the tinfoil strips for the purpose of measuring the conductivity 



