488 Dr. H. H. Poole on the Electrical 



VII. Conclusion. 



The experiments which have been described above will 

 give some idea of the possibilities and applications of Sir 

 J. J. Thomson's method of measuring explosion pressures. 

 From the results obtained in these preliminary experiments, 

 the form of the rise and fall of pressure curve in an explosive 

 mixture of gases and the nature of the explosive waves in 

 water caused by detonating some ordinary explosives have 

 been found. The law of the variation in maximum pressure 

 with the distance from the charge in the neighbourhood of 

 the explosion has been obtained. The velocity of propaga- 

 tion of the wave was found to be slightly higher than that of 

 sound. 



In conclusion I wish to express my indebtedness and 

 thanks to Sir J. J. Thomson for entrusting to me the de- 

 velopment of this method of measuring explosion pressures, 

 and for his interest during the progress of the research. My 

 thanks are also due to Professor J. 0. McLennan for his 

 constant support during the investigation, and to Mr. E. 

 Everett who was associated with the early work 



LVII. On the Electrical Conductivity of some Dielectrics. 

 By H. H. Poole, Se.D* 



AN account was given some time ago (Phil. Mag. July 

 1916 & Sept. 1917) of some tests on the electrical 

 conductivity of mica in which it was shown that the con- 

 ductivity, instead of being independent of the gradient, as 

 demanded by Ohm's Law, rose rapidly with increase of pres- 

 sure, the logarithm of the conductivity being nearly a linear 

 function of the gradient. 



In repeating the observations on mica and extending them 

 to other dielectrics the following alterations in the method 

 have been made : — 



( 1) In place of using a Wimshurst machine as a source of 

 high pressure, this is obtained by means of a transformer 

 and two thermionic rectifying valves. This arrangement 

 forms a much steadier, and more easily regulated source, 

 and is capable of supplying much larger currents. 



(2) An arrangement is used for continuously observing 

 the pressure, instead of only finding it at a certain instant as 

 previously. 



* Communicated by the Author. 



