Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
591
me will be negatively electrified till the distance of the two
iZd" is one inch, at which distance the electricity becomes no-
jaws. thin0-, and beyond it it becomes positive. If the small
globe is only four inches in diameter, the same pheno¬
mena take place, but at the distance of two inches and
five lines. .
eral When six equal globes, two inches m diameter, were
ial placed in one line in contact, and electrified, and then
bes. examined by the torsion balance, Coulomb found that the
electrical density of the Jirst was to that of the second as
148 to 100, and that of the Jzrst to that of the third as
156 to 100. When twelve similar globes were similarly
placed, the density of the first was to that of the second
as 150 to 100, and that of the first to that of the sixth as
170 to 100. When twenty-four similar globes were simi¬
larly placed, the electric density of the first was to that
of the second as 156 to 100, and to that of the tu-elfth as
175 to 100. At equal distances from the extremities of
the row the electric densities were equal, and the density
always least in the middle.
The last series of Coulomb’s experiments which we
shall notice at present, are the highly important ones
relative to the distribution of electricity between a globe
and cylinder. When the globe was eight inches in dia¬
meter, and the cylinder thirty inches long, he obtained
the following results :—
Diameter of Cylinder.
24 lines
12
2
Mean Electric Density of the
Globe to that of the Cylinder.
1 to 1-30
1 — 2-00
1 — 9-00
Hence the electrical densities of different cylinders are in
the inverse ratio of the power f- of their diameter, which
approaches very much to unity when the diameter of the
globe is very much greater than that of the cylinder.
When the globes are different, and the cylinders re¬
main the same, the electric density of the cylinders will
vary as the diameters of the globes, if their diameters are
much greater than that of the cylinder. Hence, call¬
ing D the mean electric density ot the globe, d that of
the cylinder, R the radius of the globe, and r that of
the cylinder, we have d ~ —— or a — , whenli
is much greater than r. Coulomb found the constant
. ,9
co-emcient m to be -r^.
48
rod with the point, the electricity will be rapidly discharg- Phenome-
ed from it, and will be seen streaming out from it in the ”a anrl
dark. ,
The experiments contained in the preceding section
afford a beautiful and satisfactory explanation of the ac¬
tion of points. We have already seen that the electri¬
city communicated to a cylinder is so distributed that /
the electrical density of the extremity is 2-30, while
that at the middle is 1 ; and that when the electrical den¬
sity of a globe is 1, that of a cylinder two lines in diame¬
ter and thirty inches long is 9. But we may consider
'points as cylinders of small diameter and great length,
and, following the result now mentioned, we shall find
that the electrical density at the rounded extremity of a
cylinder two lines in diameter will be 9 X 2*3 = 20,7,
while that of the globe which the cylinder touches is only
one. In order to make this plain, we have represented in pi. CCXI.
fig. 7 a cylinder or rod AB, in which the ordinates of the Fig. 7-
curve McN represent the electrical density at different
points of the cylinder, or the thickness of the stratum of
electricity at these points. The ordinate cd being 1,
the ordinates AM and BN will be 2,3. But it may be
shown, from the law of repulsion, that the re-action of the
electric fluid upon the adjacent air varies as the square of
the thicknesses of the electric strata, or as the squares of
the electric densities. Hence the squares of the ordi¬
nates cd, AM, or 1, and 2-30 X 2*30 = 5-27, will repre¬
sent the re-action at d and A, that is, the electric fluid will
have five times the tendency to escape at A, from what it
has at d.
When the point A is connected with a ball B, as in fig. Fig. 8.
8, the tendency of the electric fluid to escape at A will
be seen from the ordinates of the curve BM, the ordinate
at A being very great. We have already seen that the
ordinate AM, or the electrical density at A, is 20-7 times
as great as the electrical density at B. Hence 207 X
20-7 =: 428-49 will represent the tendency of the electri¬
city to escape from A, the tendency to escape from B
being only one. But this tendency to escape is resisted
by the air ; and as the amount of resistance varies with the
density, moisture, and temperature of the air, there will
obviously be some degree of electrical density which will
overcome that resistance. This result experience com¬
pletely confirms, for even in the common state of the air
a very great quantity of electricity is not necessary to
make its way from a pointed conductor.
This tendency of points to discharge their electricity
against the resisting air, enables us to perform some beau¬
tiful electrical experiments, in which a motion of rotation
Sect. X.— On the Action of Points, and on Electrical
Rotations.
ction of The influence of points in silently drawing off electri-
rnits. c;ty from a conductor has already been mentioned, and
also their influence in discharging electricity from any
conducting body in which they are fixed. Both these
effects are distinctly seen if a person insulates himself by
standing on a stool with glass feet, placed near an electri¬
fied prime conductor. If he takes in his hand a rod of
metal with a ball at one end and a sharp point at the other,
and holds the point at a certain distance from the conduc¬
tor, he will be able to electrify himself in consequence of
drawing the electricity from the conductor, whereas if he
holds the ball at the same distance, he will receive no
electricity at all. On the contrary, if he connect himself
with the prime conductor by a chain till he is charged
with electricity, and then throws aside the chain, he will
not be able to discharge the electricity quickly from his
body by holding out the ball, whereas if he holds out the
is effected.
Exp. 1. If one, two, or any other number of wires are Electrical
placed, as in fig. 9, so as to have beneath their centre of gra-
vity A, a hollow cup, which rests on the top of an insulated11§-
stand AB; and if the points rn, o, n, p of these wires are
made short, and are turned in the same tangential direc¬
tion ; then, if we connect them with the prime conductor
by a chain C, so as to electrify them, the electricity will
issue from each point; and as it will be resisted by the air
against which it presses, the arms will turn round in a di¬
rection opposite to that in which the electric fluid is dis¬
charged, in the very same manner as the rotatory motion is
effected in Barker’s mill. In the dark a stream of light
will exhibit the discharge of the electricity, and when the
velocity of rotation becomes sufficiently great, the four
streams will form a beautiful circle of light.
Exp. 2. The Electrical Orrery, as it is called, is found- Electrical
ed on the same principle. A spherical ball of metal S, orrery,
fig. 10, representing the sun, has its inner concave surface l’1#- t-
supported on a pivot on the top of an insulated stand CD.
From the ball S extends a wire SE, the turned-up extre-