36 Prof. Threlfall and Messrs. Brearley and Allen. [May 24,. 



quarter of a millimeter thick is charged for ten minutes with about 

 300 volts. By very careful drying we have succeeded in reducing 

 the residual charge with a film of mica 0*2 mm. thick to about 1 per 

 cent, of the original charge under similar circumstances. 



In view of the want of homogeneity in the crystalline sulphur 

 film this freedom from residual effect is noteworthy, and is perhaps 

 to be explained by the entire absence of conductivity. 



Crystalline sulphur has an electric strength which is more than 

 enough to support 33,000 volts per centimeter how much more we 

 do not know. At 75 C the specific resistance with 285 volts per 

 quarter millimeter falls to about 6'8xl0 25 C.G.S. The specific 

 inductive capacity increases slightly as the temperature rises. As 

 the temperature of the sulphur rises the conductivity increases 

 slightly up to the melting point, when there is an enormous increase. 



When a film containing about 5 per cent, of insoluble sulphur pro- 

 duced by cooling rapidly from a temperature abov? 170 C is 

 examined, it is found to have a sensible conductivity which is not 

 due to surface action, for it is not altered by fusing quartz rods into 

 the exposed part of the surface, nor by blowing air saturated with' 

 water vapour against the surface. The conductivity depends on the 

 exact composition of the mixture of soluble and insoluble sulphur, 

 but may be taken at from 10 25 to 10 26 C.G.S. units for a film contain- 

 ing from three to six per cent, of amorphous unstable sulphur at 

 ordinary temperatures. This conductivity is always greater when 

 the voltage of about 300 volts on a film a quarter of a millimeter 

 thick is first applied, or reversed. It is established that the in- 

 creased conductivity occurs after the sulphur has rested whether 

 the voltage is applied for the first time, or whether it has been 

 applied before in either direction. When the voltage is reversed 

 this effect is more strongly marked, and the conductivity only settles 

 to a steady value after a considerable time. The conduction, eifchei- 

 when the current is steady, or when it commences or is reversed,, 

 does not obey Ohm's law either for small voltages (say eight volts) 

 or large ones (say 300) when the film is 0'25 mm. thick. The 

 deviation is, however, greater at high voltages, and greater when 

 the "commencing" or "reversing" effects are taking place than 

 when the conduction is steady. The deviation is always in the 

 direction of making the conduction at high electromotive intensities- 

 greater than at low. The specific inductive capacity of a mixture of 

 soluble and insoluble sulphur is markedly higher than that of purely 

 crystalline sulphur. We have some evidence that the changes occur- 

 ring during the first few days after the film is made lead to art 

 increase of specific inductive capacity. The temperature coefficient 

 of the specific inductive capacity is positive, and of the order 2 x 10" 5 " 

 pev degree between 20 and 70 C. 



