Prof. Gr. Quincke on Electrical Expansion, 35 



mains almost unaltered. The increase of the angle cf> corre- 

 sponds, therefore, to the decrease of the force of torsion of the 



suspension-thread; and ~- is the measure of the change of 



elasticity of the thread. 



Upon discharge, the magnet and thread resume their ori» 

 ginal position. 



The decrease in the force of torsion is nearly proportional 

 to the square of the difference in electric potential of the two 

 coatings, and is greater the less the thickness of the walls of 

 the glass thread. 



"With a quantity of electricity 20 in the Leyden jar used 

 (of six jars), and a thickness of glass of the suspension-thread 



0-1 millim., ^ was 0-00055 for flint-glass and 0'002 for 



Thuringian glass. 



An indiarubber tube, gilded on the outside and filled with 

 water, showed with the same Leyden jar about the same 



change -— as the far thinner thread of Thuringian glass. 



28. A strip of mica 840 millims. long, 30 millims. broad, 

 and 0*04 millim. thick, covered with gold leaf on one side and 

 carrying a magnetic bar, in the same way gave an increase of 

 torsion of -^ of the original value when the coated side was 

 connected with the earth, and the uncoated side electrified by 

 means of a metallic comb connected with a Holtz's machine 

 which was passed along it. A band of gutta percha of like 

 dimensions showed under the same circumstances an increase 

 in torsion of 0*0031 6 of the original value. 



29. Similar results were obtained in experiments in which 

 the torsion of the glass thread was balanced by the torsion of 

 a metallic wire. 



Electric Expansion of Fluids. 



30. The expansion produced by electric force may be 

 observed with liquids as well as with solids by using a volta- 

 meter with platinum electrodes filled with the liquid, the 

 delivery tube of which is replaced by a vertical capillary 

 tube. The apparatus is maintained at a constant temperature 

 by means of melting snow. An increase of volume is 

 observed when the platinum electrodes are connected with the 

 coatings of a charged Leyden jar. When the battery is dis- 

 charged the fluid returns to its original position in the capil- 

 lary tube. 



With fluids which are good insulators, such as sulphide of 

 carbon and ethereal oils, the Leyden battery retains its charge 



D2 



