84 Prof. G. Quincke on Electrical Expansion. 



with Thuringian glass. There remains, as with the changes 

 of volume of the thermometer condenser, a residual displace- 

 ment in the same direction as the original displacement, which 

 gradually disappears. 



21. With the same glass-thread electrometer, the displace- 

 ment is proportional to the square of the difference of electric 

 potential of the two coatings. 



22. The residual displacement increases with the difference 

 of electric potential and with the conductivity of glass ; it 

 seems therefore to depend, like the residual expansion of the 

 the thermometer condenser, on the polarization of the glass. 



23. By means of a suitable key the coatings of the thermo- 

 meter condenser could be put into communication, first with 

 the poles of a bichromate battery of 44 cells, and then with a 

 sensitive mirror-galvanometer. The deflection of the galva- 

 nometer was then proportional to the electric capacity of the 

 thermometer condenser. When the temperature rises, the 

 change of volume corresponding to a given difference of poten- 

 tial is proportional to the increase of electric capacity for the 

 same thermometer condenser. 



24. Similarly, the deflections of a glass-thread electrometer, 

 made of the same kind of glass as a thermometer condenser, 

 increase in the same proportion as the electric capacity of the 

 condenser. A temperature-increase of 1° C. corresponds to 

 an increase of deflection or of capacity of about 0*003 of the 

 original value for flint-glass and about 0*012 for Thuringian 



glass. 



25. The greater the difference of electric potential and the 

 higher the temperature of the glass, the more quickly do the 

 deflections of a glass-thread electrometer take place. Flint- 

 glass acts more slowly than Thuringian glass. 



Changes of Elasticity caused by Electricity. 



26. The elasticity of flint-glass, Thuringian glass, and caout- 

 chouc is diminished by electricity ; but that of mica and gutta- 

 percha is increased. 



27. A magnetic bar was hung at the lower end of a hollow 

 glass thread silvered inside and outside, so that the magnetic 

 axis was nearly at right angles to the magnetic meridian. The 

 moment of the magnetic force was then equal and opposite to 

 the moment of torsion of the glass thread corresponding to the 

 angle (/> through which it had been twisted. 



A vertical mirror was attached to the magnetic bar, whose 

 position was observed with a telescope and scale. When the 

 coatings of the glass thread are connected with the coatings 

 of a charged Leyden jar, the angle of torsion becomes 

 greater by the amount A^>, while the magnetic moment re- 



