Caven- 
dish’s 
electrical 
988 
of other matter with a force varying inversely as some 
less power of the distance than the cube. Common 
theory and Matter repels its own particles and attracts electric 
experi- 
ments, 
(872.) 
He makes 
an arti- 
ficial 
torpedo. 
particles according to the same law. The limitation 
as to a power below the inverse cube of the distance 
is necessary, since were the decrease of force more 
rapid, a particle would not be sensibly affected by the 
repulsion of any portion of the fluid except what was 
placed close to it. The hypothesis of Cavendish and 
his mode of reasoning from it were in general the 
same as those of Zpinus; but Cavendish was not 
aware of the researches of the Swedish philosopher 
until his own memoir was completed. The number 
of facts accurately ascertained concerning electricity 
was at that time too small to admit of very precise 
numerical comparisons, but the ordinary cases of at- 
traction, repulsion, and induction, were perspicuously 
explained by the theory; and had the inverse square 
of the distance been assumed (as it very safely might 
have been) to represent the law of diminishing re- 
pulsion, several of the theorems would have as- 
sumed a much more definite character, as was 
shown by Robison. Cavendish first demonstrated 
in his paper of 1771 that electricity must be 
confined close to the surface of a spherical body. 
This memoir also includes a correct theory of 
the Leyden phial, a just approximation to the law 
of attraction as the inverse square of the distance, 
a theory of conduction, and of the distribution of 
electricity on insulated conductors placed at a dis- 
tance but connected by a fine wire or electric canal ; 
and it was only the prelude to other researches never 
published, but of which some remarkable fragments 
exist amongst his manuscripts. Professor William 
Thomson, who has partly examined these, informs 
me that they contain the experimental solution of 
problems such as the following: “To compare the 
quantities of electricity on a spherical conductor, 
and a plane disk of equal diameter connected by a 
long conducting wire.” Cavendish found that the 
sphere holds 1:57 times the electricity of the plate, 
a result exactly coincident with the deductions of 
theory. 
I may here (for the sake of biographical connection) 
mention Cavendish’s paper on the Torpedo, as a re- 
markable instance of the explanation of an obscure 
natural phenomenon, by the analogous effects of an 
artificial imitation. The experiments of Walsh on the 
Electrical Fishes have been cited in the preceding Dis- 
sertation, Cavendish undertook the bold task of prov- 
ing, that all the external effects of the shock under 
varied circumstances, might be reproduced by a com- 
bination of Leyden jars duly protected. He made an 
artificial torpedo, consisting of 49 jars in a frame 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. VI. 
covered with leather, and he succeeded in obtaining 
shocks in air as in water, exactly comparable to those 
obtained from the live fish by Walsh. But perhaps the 
most striking part of the paper is the incidental men- 
tion of several laws of electricity then certainlynew,and 
which he had deduced from experiment. Thus,he af- 
firms the conducting power of iron for electricity to 
exceed 400,000 times that of distilled water, which he 
states to be equivalent to the fact, that a conductor of 
equal diameter will transmit as much electricity if the 
iron be 400,000 times longer than the water—a law 
conformable to that of Ohm. He farther estimates 
the conducting power of sea-water at 100 times, but 
of saturated brine at 720 times that of pure water. 
Again, the quantity of electricity required to raise the 
charge on different jars or plates to the same inten- 
sity he finds to be directly as the area of the coating, 
and inversely as the thickness of the plate, and he 
applies this just conclusion with great ingenuity to 
explain the surprising power of the torpedo’s shock 
by the extreme fineness of the membranes separating 
the columns of the electrical organs. 
Couxoms (born 1736, died 1806") was a person of _(873.) 
less genius and less mathematical attainment than 
Cavendish, yet he had very considerable geometrical 
Coulomb’s 
electrical 
experi- 
ability and much facility in applying it to the results ments. 
of experiments, which he conducted with the greatest 
ingenuity and accuracy. In the latter respect he has 
seldom been surpassed. His methods, and even his 
numbers, are still, after a lapse of more than half a 
century, in many cases the best we can quote. 
Like Cavendish, he was devoted to quantitative esti- 
mations of phenomena. 
His two greatest inventions were the balance of _ (874) 
torsion and the proof plane. In the course of his;,)¢. or 
strictly mechanical researches (which, as we have torsion 
seen in the chapter on Mechanics, Art. 389, &c., were and the 
numerous and important) he ascertained the laws of 
torsion. Within the limits of perfect elasticity, he 
found that the force is as the angle of torsion of the 
wire or fibre, and inversely as its length. An almost 
indefinite minuteness may thus be attained in the mea- 
sure of forces which may be balanced by the elastic tor- 
sion of a wire. We have seen (Astronomy, § 1, Art. 
156) how it was applied by Michell and Cavendish to 
measure the gravitation of bodies, Coulomb's inyen- 
tion dates at least from 1784, By means of it he 
established (in a different and perhaps more satisfac- 
tory way than had been done by Robison in 1769) that 
the electric and magnetic forces vary according to the 
Newtonian law;? and with the aid of the “ proof 
plane” he obtained exact measures of the electric 
tension on any part of an excited body. The “ proof 
plane” consists of a small gilt disk with an insulat- 
1 For a farther account of Coulomb, see the chapter on Mechanics, § 2. 
® Besides this law, Coulomb experimentally proved two others of great importance :—l. That the electricity of an elec- 
trified conductor resides wholly on its surface, 
by the presence of other external excited bodies, 
2. That the interior of such a conductor is in a condition absolutely undisturbed 
proof 
plane. 
