406 Mr. Ervin S. Ferry on the Specific 



a Babinet Compensator, so that the thickness of sulphur intro- 

 duced could be varied at pleasure. 



By comparing the force of attraction exerted on a small 

 sphere of a dielectric when placed in a field of known intensity 

 with the force exerted on a metallic sphere of equal size placed 

 in the same field, Boltzmann* has measured the dielectric 

 constants of several amorphous solids, and also the dielectric 

 constants of sulphur in the direction of the optical axis. 



The law of the proportionality between the force of attrac- 

 tion exerted between two plates charged to different potentials 

 to the specific inductive capacity of the dielectric separating 

 them, has recently been utilized by several experimenters to 

 determine dielectric constants of electrolytes. " Rosa f measured 

 the force exerted between a fixed and a movable electrode 

 immersed in the liquid dielectric under investigation with a 

 form of apparatus similar to that used by Boltzmann. One elec- 

 trode was stationary and one was suspended on the end of a light 

 horizontal rod by means of a thin fibre, in a manner analogous 

 to the Cavendish balance. LefevreJ substituted a beam- 

 balance for the torsion balance. One pan was replaced by 

 an electrode, and the other electrode was fixed a short distance 

 beneath. Trouton and Lilly § have suggested a method con- 

 sisting in measuring the force exerted by a charged condenser 

 tending to draw a dielectric in between the plates. The 

 apparatus consists of the alternate quadrants of a quadrant- 

 electrometer charged to different potentials, the other two 

 quadrants being removed. The specimen whose dielectric 

 constant is to be determined is made in the form of an elec- 

 trometer-needle and is suspended between the quadrants. 



J. J. Thomson || and A. Perot H have found the constants 

 of various dielectrics by measuring the wave-length of the 

 electrical vibrations given out by a discharging condenser 

 when the plates are separated by air and when they are sepa- 

 rated by the dielectric under investigation. The ratio of the 

 capacity of the condenser when air separates the plates to the 

 capacity when another dielectric is inserted, equals the ratio of 

 the squares of the wave-lengths produced when the condenser 

 is discharged under the two conditions. So that by measuring 

 the capacity of the air-condenser and the wave-lengths of the 

 electrical vibrations in the two cases, the specific inductive 



* Sitz. Akad. Wiss. Wien, lxviii. p. 81 (1873) ; lxx. p. 342 (1874). 



t Phil. Mag. [5] xxxi. p. 188 (1891). 



| Journal de Physique, June 1892. 



§ Phil. Mag. [5] xxxiii. p. 529 (1892). 



|| Proc. Roy. Soc. xlvi. p. 292 (1889). 



H C. JR. cxiv. p. 1528 (1892). 



