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X. On the specific Inductive Capacities of certain Electrical Substances. 
By W. Snow Harris, Esq., F.R.S., 8$c. 
Received May 21, — Read June 9, 1842. 
i. The unrivalled series of Researches in Electricity with which Dr. Faraday 
has enriched the pages of the Royal Society’s Transactions, have greatly extended 
our field of view in this wonderful department of natural knowledge. 
The doctrine of specific inductive capacity advanced in these profound researches, 
has very considerable claim to attention, being both a novel and important feature of 
electrical action. I have been hence led to some further examination of it, and, 
from the results obtained, I am not without hope that a brief account of them may 
be worthy the notice of the Royal Society. 
2. If a given measured quantity of electricity be deposited on different insulating 
substances of the same thickness, and having metallic coatings of the same extent, 
the intensity of the charge, as shown by an electrometer, will greatly vary. I found 
the differences in some cases to be so great as twenty-five to one. Thus, in one in- 
stance, the intensity of the charge sustained by induction through air being 25°, the 
intensity of the same charge sustained by induction through lac only amounted to 1°. 
An experimental examination of this question, however, demands very considerable’ 
caution, since a small degree of conducting power, or dissipation of the charge, or a 
partial absorption of electricity by the superficial particles of a given substance, would 
at once diminish the apparent intensity; whilst, on the other hand, any subsequent 
evolution of the quantity absorbed would, if added to the quantity subsequently de- 
posited, tend to increase it. It is hence essential that experimental processes for the 
detection and measurement of specific induction should be such as to admit of being 
carried out in the least possible time under conditions of very perfect insulation. 
3. Such processes I have endeavoured to supply in the following experimental ex- 
amination of this interesting subject ; they may be thus briefly stated. 
The given substance to be examined being cast into a circular plate of a foot in 
diameter and four-tenths of an inch thick, by means of a mould formed of two 
pieces of polished marble, and an intermediate ring of brass, coatings of tin foil 
six inches in diameter were applied to each of its surfaces, so as to leave an in- 
sulating edge of three inches wide. Plate XIII. fig. 1. represents a plate thus pre- 
pared, in which a b is the plate, and c the central coating. When a dielectric medium 
of air was required, the opposed coatings were made of wood, about three-tenths of 
an inch in thickness, covered with tin foil ; and were fixed at the given distance by 
