THE ELECTRIC AND LUMINrFEROUS MEDIUM. 
293 
cease to be horizontal; the upper plate is then to be rotated until horizontality 
is again obtained, as may be tested very exactly by reflexion of a beam of light; 
then the ratio of the tangents of the inclinations of the plates will be that of the 
inductive capacities of the media. The method would also apply to solids, as we 
might employ a prism of the material, over a horizontal face of which a sheet of a 
fluid dielectric is spread, and observe the deviations from horizontality of the upper 
surface of this sheet. An equivalent arrangement has been actually employed for 
solids by Perot, '=• who however adjusted the plates to uniformity of the electric field 
by the electric test that translation of a small piece of solid dielectric in the field 
between them should not aftect the capacity of the condenser. 
78. Experiments on Electric Traction and Change of Pressure in Fluids.—The 
direct expeiimental examination of the material forcives of polarized media is neces¬ 
sarily confined to fluids, for in the case of solids the strains produced by them could 
hardly be disentamgled from the intrinsic changes of configuration due directly to the 
polarization. The field for fluids has been very fully explored by QuiNCKE.t The 
inductive capacity of the fluid dielectric of a horizontal condenser was first determined 
by direct electrical measurement. The attraction between the plates was then 
weighed. Then, using a wide cylindrical air-bubble extending across the space 
between the plates, and connected through an aperture in the upper one with a 
manometer, the increase of air-pressure in the bubble produced by charging the 
condenser was measured. As half-way between the jilates the capillary interface 
between air and liquid lies along the lines of force, there is by the previous formula; 
(§ 37) no true surface traction on that part of the interface; so that the indication of 
the manometer would give exactly the change of pressure in the liquid due to the 
electric excitation, were it not that the different electric conditions over other parts 
of the interface change the value of its curvature and so introduce a capillary change 
of pressure. Finally, employing a fiat bubble of air resting against the upper pla'te 
alone, and maintaining the pressure in it constant, the change of curvature of its 
lowest part produced by the electrical excitation was measured by the optical method; 
the surface tension operating through this change of curvature must balance exactly 
the direct traction on the surface and the change of pressure in the liquid below it. 
To compute these, we notice that the line of force through the middle of the bubble 
* P^ROT, ‘ Comptes Rendus,’ 1891; quoted bj Drude, ‘ Pbysik de.s Mothers,’ p. 299. The conjugate 
condensing system, in which namely the line.? of force and tlie line.s of equal potential are interchanged, 
so that the plates are now bent according to the law of tangents where they cross the surface of the 
liquid, has recently been brought into requisition by Pellat (‘Annales de Chimie,’ 1895), in order to 
derive the law of the traction on a dielectric interface from the expression for the energy of the dielectric 
system. As m these cases the field of force is uniform in both media, the bodily part of the mechanical 
force vanishes, and the interfacial traction proper thus constitutes the whole forcive: but that would 
not generally be so. 
t G. Quincke, ‘ Wied. Ann.,’ 19, 1883; or an abstract in the paper below cited, ‘Roy. Soc. Proc ’ 52 
1892, pp. 59-62. •> . 
