228 Mr. T. Gray on the Electric Resistance 



than is proper to that temperature has taken place. The second 

 method gives during the heating pretty nearly what is believed 

 by some to be the true resistance ; but it also shows whether 

 more or less polarization takes place when the temperature is 

 high than when it is low. It therefore gives some indication 

 of the effect of the current flowing constantly through the 

 glass while its temperature is being changed. 



The results which I have obtained agree with the well-known 

 result that the resistance of a piece of glass diminishes as the 

 temperature is raised ; but they also show that the resistance 

 varies more slowly when the glass is cooling than when it is 

 being heated, and that a piece of glass may have its resistance 

 greatly increased by being slowly raised to a high temperature 

 and slowly cooled. There seems, therefore, to be what may 

 be called a permanent change in the quality of the glass, 

 which takes place coincidently with a temporary or quasi- 

 elastic change in the quality; and a comparison of the results 

 obtained when the glass was cooling with those obtained when 

 it was being heated, showed that some such equation as that 

 which I have stated above probably expresses this latter change. 

 Whether the former change is really permanent or not (that 

 is, whether the glass retains this increase of specific resistance 

 at low temperatures) I am not yet in a position to say with 

 certainty ; but, from observations which I have made, I am 

 inclined to think that slow changes take place in glass, tending 

 to raise its conductivity and also its specific inductive capacity. 



The following table, the results in which were obtained by 

 the second method described above, gives the resistance at four 

 different temperatures, in ohms per cubic centimetre, of a 

 test-tube during four successive heatings and coolings. H 1; H 2 , 

 H 3 , H 4 , Ci, C 2 , C 3 , C 4 indicate the first, second, third, and 

 fourth heatings and coolings respectively. 



Temperature, 



H r 



C x . 



H 2 . 



C 2 . 



O 



60 

 100 

 150 

 200 



20 X 10 10 

 79X10 8 

 23X10 7 

 18X10 6 



24X10 10 



lOxlO 9 



30X10 7 



18 x10 s 



25 X 10 10 

 81xl0 8 

 27X10 7 

 20xl0 6 



44X10 10 

 22xl0 9 

 50X10 7 

 30X10 6 



Temperature. 



H 3 . 



c 3 - 



H 4 . 



C 4 . 



o 



60 

 100 

 150 

 200 



40xl0 10 

 18X10 9 

 50X10 7 

 67X10 6 



1 27X10 11 

 11X10 10 

 30xl0 8 

 70xl0 6 



13xlO u 

 38X10 9 

 12 x10 s 

 16xl0 7 



70X10 11 

 41X10 10 

 14X10 9 

 73X10 7 



