Electrical Conductivity of Mica in In' ease Fields. 127 



This effect, though looked for, was never detected in the 

 galvanometer experiments, showing that this reverse current 

 does not increase with the field to at all the same extent as 

 the direct current. 



The low value found for the specific resistance in weak 

 fields may be due to a permanent change in the mica caused 

 by the intense fields to which it had been subjected. On 

 the other hand, it seems equally probable that large varia- 

 tions in the resistivity may occur in different specimens. 

 Some experiments on the insulation resistance of the standard 

 condenser showed that the resistivity of the mica used in 

 its construction was not less than 5 x 10 16 . This value was 

 obtained when the potential difference across the condenser 

 was anything between 20 and 30 volt«. With smaller 

 pressures considerably larger values for the resistance were 

 obtained, but in all cases the interpretation of the readings 

 was rendered difficult by soakage effects. 



Meaning of the Results obtained. 



No evidence has been obtained in favour of a theory of 

 -electric polarization analogous to the molecular theory of 

 magnetism, if we except the small hysteresis effect observed,. 

 as no certain variation of k with X has been established, 

 even in the most intense fields. On the other hand, the 

 results do not furnish any real evidence against such a 

 theory. According to Sir J, J. Thomson's doublet-chain 

 theory *, polarization and conduction are due to the forma- 

 tion of chains of electric doublets under the influence of the 

 field. If there are N doublets per c.c. arranged in chains 

 and the electric moment of each is M, the total electric 

 moment per c.c. will be NM. Now, taking the highest of 

 the figures given, the value found for this moment was about 

 6240 E.S.U. If we assume that each doublet consists of 

 a + and a — charge each 4*8 X 10~ 10 E.S.U. at a distance 

 10" 8 cm. apart, the value of M will be 4*8 x 10" 18 , and there- 

 fore N will be l'3x 10 21 . Taking the formula of Muscovite 

 as K 2 O,3Al 2 3 ,6Si0 2 ,2H 2 O, there are about 2*3 x 10 21 mole- 

 cules per c.c, and each contains 42 atoms. It is there- 

 fore probable that, even in the most intense fields employed, 

 the number of doublets arranged in chains formed only a 

 small fraction of the whole, and so it is not surprising that 

 no evidence of saturation was obtained. 



* J. J. Thomson, ' The Corpuscular Theory of Matter,' p. 86. Also 

 Phil. Mag. July 1915. 



