On the Electrical Properties of Different Kinds of Glass. 201 



Measurement of Residual Tmst of Glass Fibres, 



The object of these experiments was to compare fibres of the glasses, 

 on which the electrical experiments already described had been carried 

 out, as regards imperfection of their torsional elasticity. 



The figure shows a sketch of the apparatus used. A long cylindrical 

 glass tube (a), about 23 inches in length and 2 inches in diameter, was 

 fixed permanently in a vertical position in a wooden frame, as indi- 

 cated in the diagram. A tightly-fitting cork closed the upper end of 

 the tube, and through a narrow slit in this passed a rectangular piece 

 of copper foil, from the centre of the lower edge of which projected a 

 short tag of the foil. This tag was quite rigid, and carried attached 

 to it with shellac cement the glass fibre, care being taken that the fibre 

 was so placed that it hung vertically along the axis of the tube, when 

 the cork was fixed in position. The lower end of the fibre was 

 attached in a similar manner to a cross-piece, c, weighing about 

 1 gramme, and having a length of about If inches. The fibre was 

 attached to the upper end of the cross, and to the lower end was fixed 

 a small mirror. 



The lower end of the glass tube fitted into a groove cut in a wooden 

 sole-plate, capable of being rotated about a vertical axis, which 

 coincided as nearly as possible with the axis of the tube, that is, with 

 the position of the fibre. Two brass pins (&) were fixed vertically at 

 two points in the sole-plate within the tube, and, projecting upwards, 

 stood one on one side the other on the other side of the horizontal 

 arm of the cross-piece. By turning the sole-plate round, any required 

 twist could be given to the fibre, since the tube was held fast in its 

 supporting frame. 



A lamp and scale set in front of the mirror enabled the position of 

 the lower end of the fibre to be observed. 



