390 Prof. W. M. Thornton on the Polarization of 



and it is desirable to know the pressure in the vessel, and 

 yet keep the gas pure, it would be desirable to have a gauge 

 which would not introduce any impurity. 



If the inference made above as to the validity of the 

 McLeod gauge measurements on gas pressure is allowed, 

 then we can say that reliance may be placed upon the measure- 

 ments of pressure from decrement measurements in the 

 apparatus used in this investigation. This method need in- 

 troduce no mercury vapour, but it takes account of all that 

 is in the vessel." Moreover, a discussion similar to that used 

 for mercury vapour will show that in the case of oxygen the 

 decrement, corresponding to a certain gas pressure, will be 

 about four times as great as it is in the case of hydrogen. 

 In the case of oxygen, therefore, a pressure of O'OOOOl mm. 

 should be measured with an accuracy of from five to ten 

 per cent. This would indicate an absolute error of less than 

 0*000,001 mm. 



The investigation is now being extended to the case of 

 oxygen and nitrogen. The data obtained by using these 

 oases, besides showing- whether their behaviour is like that 

 of hydrogen, should give some more information regarding 

 the quantity /3, which enters the foregoing discussion. 



Jefferson Physical Laboratory, 

 Cambridge, Mass. 



XXXIX. The Polarization of Dielectrics in a Steady Field 

 of Force. By W. M. Thornton, D.Sc, D.Eng., Professor 

 of Electrical Engineering in Armstrong College, Neivcastle- 

 on-Tyne *. 



Contents. 

 Section Page 



1 . Introduction 390 



2. Experiments on Quartz 392 



3. Experiments on Elint Glass 396 



4. Final values Independent of Intensity of Field. . . . 398 



5. Rate of Polarization of Various Dielectrics 399 



6. Measurement of Specific Resistance 402 



7. Nature of Slow Polarization 404 



1. Introduction. 



THE slow polarization of a dielectric ellipsoid suspended 

 in a steady field of force can be followed by observing 

 from time to time the period of small swings about the line 

 of the field j". In a perfect insulator the polarization reaches 



* Communicated hy the Physical Society : read January 21, 

 t Hoy. Soc. Proc. A. vol. lxxxii. p. 422 et seq. 



