Note 26: influence of temperature on glass 417 



could be placed in an iron pot filled with iron-filings. A thermometer was put 

 into the filings; a lamp was placed under the pot; and the whole was supported 

 upon glass. 



The wire which entered one end of the tube was electrified by a machine, 

 a cork ball electrometer was hung on the other, and a small wire, reaching to 

 the floor, was tied round the tube between the pot and the electrometer, in 

 order to carry off any electricity that might fun along upon the tube. 



"Before the heat was applied, when the machine was worked, the cork 

 balls separated at first upon the principle of the Leyden phial. But after the 

 middle part of the tube was heated to 600, the corks continued to separate, 

 though you discharged the electricity by touching the wire, the electrical 

 machine continuing in motion. Upon letting the whole cool, the effect remained 

 till the thermometer was sunk to 400." 



Experiments on the conductivity of glass at different temperatures have 

 been made by Buff*, Perryf, and Hopkinson J . 



Hopkinson finds that if B is the specific conductivity divided by the specific 

 inductive capacity and multiplied by 477-, then for 



glass N. 2, log B = 1-35 + 0-04150, 

 glass N. 7, log B = 4-17 + 0-02830, 

 where 6 is the temperature centigrade. 



Glass N. 2 is of a deep blue colour; it is composed of silica, soda, and lime. 



Glass N. 7 is "optical light flint," density 3-2, composed of silica, potash, 

 and lead; almost colourless, the surface neither "sweats" nor tarnishes in the 

 slightest degree. This glass at ordinary temperatures is sensibly a perfect 

 insulator. 



The conductivity of glass when heated makes it very difficult to determine 

 its capacity as a dielectric. It appears from the experiments of Hopkinson on 

 glasses of known composition, that the glasses made with soda and lime conduct 

 more, and are also more subject to "electric polarization " and "residual charge " 

 than those made with potash and lead. 



Both the conductivity and the susceptibility to residual charge increase as 

 the temperature rises, and this makes it very doubtful whether the apparent 

 increase of dielectric capacity, which was observed by Cavendish and also by 

 recent experimenters, is a real increase of the specific inductive capacity, or 

 merely an effect of increased conductivity. 



The experiments of Messrs Ayrton and Perry on wax at different tem- 

 peratures would seem to indicate a real increase of dielectric capacity, as well 

 as of conductivity, as the temperature rises up to the melting point. During 

 the process of melting the capacity decreases and at higher temperatures 

 begins to increase again, but the conductivity continues to increase as the 

 temperature rises. 



* Annalen der Chemie und Pharmacie, xc. (1854), P- 2 57- 



t Proc. R. S. 1875, p. 468. } Phil. Trans. 167 (1877), P- 599- 



Phil. Mag. August, 1878. 



c. p. i. 27 



