166 
MR. SNOW HARRIS ON THE SPECIFIC INDUCTIVE CAPACITIES 
means of three small supports of shell-lac, cemented to the circumferences, as repre- 
sented by fig’. 2, in which c d represent the coatings, and a b two of the shell-lac sup- 
ports. In some instances the plates were opposed to each other at the extremities of 
insulating rods of glass, as represented in fig. 8. Plate IV. of the Philosophical Trans- 
actions for 1839. 
4. Fluid dielectric media were examined by means of the arrangement represented 
in fig. 3, in which mn is a sort of glass bowl having a contracted opening and neck at 
h. This opening is closed by a fine piece of cork, so as to admit of a conducting wire 
i passing through it fluid tight ; cd are the circular coatings just described, the under 
one, d, being screwed on the end of the wire i. The whole is supported on a conve- 
nient open frame ; and the fluid to be examined is poured into the glass bowl, so as to 
completely surround and fill the space between the circular metallic surfaces cd. An 
inverted lamp glass shade may be employed with advantage for this purpose. 
5. It will be immediately seen that if under any of these conditions one of the 
coatings, c, fig. 4, be connected with the disc m of the electrometer E*, we may deter- 
mine by an easy and direct experiment, the three following elements necessary for 
the elucidation of the question under consideration. First. If we insulate the whole 
system on a glass rod k, and deposit a given measured quantity of electricity on the 
coating c, we may determine the intensity of that quantity as expressed by the elec- 
trometer, taking the whole as free charge. Secondly. By connecting the under plate 
d with the electrometer, and charging the upper plate c with a given measured quan 
tity, we may determine in a similar way the direct induction between the plates in 
degrees of intensity, or free charge shown by the electrometer. Thirdly. By con- 
necting the under coating d with the ground, and charging the superior plate c with 
a given measured quantity, we may determine in degrees of the electrometer the pro- 
portion of the charge uncondensed by the uninsulated plate d, that is to say, we may 
measure the intensity of the charge under the ordinary conditions of the Leyden 
experiment. In this way, as is evident, we may examine any dielectric medium, 
whether solid, fluid or gaseous, contained between the metallic coatings c, d, and com- 
pare their respective influences over the degree of induction which takes place through 
them, between the coatings c, d. 
6. I have called the degree of intensity expressed in terms of the electrometer, free 
charge, for the sake of perspicuity, and in order to distinguish that portion of the 
charge, whatever it be, which is active on the electrometer, from that portion which 
is condensed by induction. For similar reasons I have called the action of the 
charged on the neutral plate, direct induction, in contradistinction to the con- 
densing action of the neutral on the charged plate, which I term the reflected, or in- 
direct induction. It must, however, be understood that these terms are merely em- 
ployed as expressing conveniently the different actions to which I shall have occasion 
to refer, and that they are limited to the definitions just given-f-. 
* This instrument is described in the Philosophical Transactions for 1839, Part II. p. 215. 
t Philosophical Transactions for 1829, p. 219. 
