VELOCITY OF ELECTRICITY—EXPERIMENTS—THE ELECTRIC EGG—FORCE OF THE
ELECTRIC SPARK.
We are now acquainted with many facts concerning electricity, and have
seen that electrical phenomena can be produced by the Electric Machine
and the Leyden Jar. (An insulating stool—a stool with glass legs—is
a very desirable adjunct for those who wish to experiment with the
machine). Glass is a great insulator, or non-conductor, as a Russian
philosopher found to his cost. He had an iron lightning-conductor from
his house into his room, the end not connected with the earth but with
a glass. One day the lightning came down the rod and reached the glass;
had a communication been made with the earth by a chain, or directly,
no mischief would have ensued. As it happened, however, the current was
checked by the glass, and immediately darted towards him; it struck
him in the head, and killed the poor man on the spot. If no insulating
stool were used, the body charged would be discharged upon contact with
the ground.
The velocity of electricity is very great, and experiments have
frequently been made. Wheatstone undertook to ascertain the speed
of the electric fluid, and the instrument he employed he called a
“Chronoscope.” He caused a mirror to revolve with enormous velocity,
and measured the speed by the vibrations of air, which produced a
certain note by the same motive power. (We know already that certain
notes are produced by a certain number of vibrations per second.)
Wheatstone’s Chronoscope consisted of this mirror, in front of which
was placed a circular block of wood, in which, in a row, were set six
wires carrying small knobs; round these and over the wood he put an
insulating varnish. A Leyden jar was connected outside with the first
knob; between the second and third a quarter of a mile of copper wire
was coiled, and a like length of wire between the fourth and fifth; the
inside of the Leyden jar was then connected with the last knob, and the
spark passed; it ran from knob to knob over the long coils of wire. If
all the flashes over the wire and knobs occurred simultaneously the
mirror would show them side by side; if not, as the mirror turned a
trifle, the difference would be observed. The mirror did show a slight
retardation in the passage of the flash, and from certain measurements
and calculations Wheatstone estimated the velocity of the spark to be
288,000 miles a second. This rate will carry electricity round the
earth in about a twelfth of a second, a rate Puck never dreamed of when
he promised to “put a girdle round the earth in forty minutes.”
But it appeared from investigations subsequently made that it was not
possible to express the velocity of electricity with any certainty,
and a number of experiments were made as per the following table, with
very different results. Sir William Thomson and Faraday endeavoured
to account for these stupendous discrepancies, and the principles of
retardation of electricity were established. The differences are shown
below:—
Nature of Velocity per
Wire. Second.
Wheatstone’s Experiment in 1834 Copper 288,000
Gonella and Fizeau {Copper 111,834
{Iron 62,130
Mitchell Iron 28,331
Walker Iron 18,639
Gould Iron 15,830
Astronomers in Greenwich Copper 7,600
“ Brussels Copper 2,700
Result in Atlantic Cable, 1857 Copper 1,430
“ “ 1858 Copper 3,000
To account for the comparatively low velocities of the cables, Faraday
proved that they act very much as a Leyden jar acts; that is, it takes
time to fill, as it were, and to discharge them, the wire coating of
the cable in air acting like the outer coating of the jar or the water
in the case of an immersed cable, and the retardation observed is
owing to resistance of conduction, and depends upon the way in which
the electrical impulses traverse the wire. “There is a long, gradual
swell, and still more gradual subsidence of the electric current, and
the length of time that elapses between the initial impulse and the
attainment of maximum strength, is proportional to the square of the
length of the line.”
The duration of the electric spark has been calculated at the 1/24000
part of a second, but Professor Tyndall regards this as the longest
or nearly the longest time it is perceptible; the _shortest_ time
is almost inconceivable. The brightest portion of a spark has been
ascertained to last only forty-six millionths of a second, and certain
experiments were made to ascertain the actual duration with various
numbers of Leyden jars. It was discovered by Messrs. Lucas and Cuzin,
by an application of the Vernier, with batteries consisting variously
of two to eight jars, and obtained the following results[14]—
Duration in
No. of Jars. millionths of a Second.
2 26
4 41
6 45
8 47
“So,” adds the writer, “the duration (of spark) increases in proportion
with the number of the jars. It increases also with the striking
distance, but is independent of the diameter of the balls or globes
between which the spark strikes.”
Many examples might be given of the _spark discharge_ of the electric
current. This form is seen in the blue line extending between the knob
of a machine and the hand, and the duration of a spark with a jar
charged with an induction coil is stated by Professor Rood to vary, but
the brightest portion with a jar of 114 square inches only existed for
the 175th _billionth_ of a second, and with a smaller surface was much
shorter. Such a spark may be conducted to a plate of gunpowder and will
not ignite it, because the time of the duration of the “fire” is not
sufficiently long; the powder will be scattered, but not ignited. If,
however, a partly non-conducting medium be interposed between the jar
and the powder, so that the spark be retarded a little, the gunpowder
will be fired.
While speaking of electric discharge we may remark upon the beautiful
effect of lightning. These discharges are sometimes miles long, and by
the return stroke from the cloud may kill a person a long way from the
actual discharge. This phenomenon was illustrated by Viscount Mahon in
1779, in a very interesting book on the principles of electricity.
There are different ways in which the electric discharge shows itself.
We have spoken about the spark discharge which, however, is found to
present very different appearances in varying conditions. Professor
Faraday proved that the colour of the electric sparks showed in air,
when obtained with brass balls, the intense light and blue colour so
familiar to all. In nitrogen they are even bluer. In oxygen again the
sparks are much _brighter_ than in air, but not so _brilliant_. In
hydrogen they become crimson, but the sound is almost inaudible because
of the physical character of the gas. In carbonic acid gas they are
almost the same as in atmospheric air, only more irregular. In dry
muriatic gas they are nearly white and very bright. In coal gas the
colours vary—sometimes being green and sometimes red. Occasionally the
same spark will be red and green at different extremities, and even
_black_ portions have been observed. The density and pressure of the
atmosphere has been proved to exercise considerable influence upon the
spark discharge.
The “Brush” discharge is shown in “a series of intermittent discharges
which appear continuous.” This discharge assumes the shape of a fan.
“It is accompanied with a low chattering sound,” which is the result of
the separate and continuous discharges, and Faraday also demonstrated
that its effects varied according to the medium in which it was
exhibited.
The effect of the air pressure on electricity may be observed in the
following way:—
If we pass a spark through rarefied air by an apparatus known as the
Electric Egg, we may obtain many curious effects. The “egg” consists
of a glass globe, through which enter two rods with a knob upon each
inside end. The upper rod is moveable, and held in its place by a “cap”
like the lower rod. There is a stop-cock in the lower cap, so that the
egg may be fastened to any plate or stand. When the egg is filled with
air, the electric spark passed into the glass globe has the usual
appearance, but as the air is gradually rarefied by an air-pump, the
spark assumes beautiful forms and colours. As the exhaustion continues,
however, we shall find the spark decreasing in brilliancy, and finally
the spark will cease to be visible. It thus is shown that the colour
of the spark depends upon the gaseous medium and on the material of
the conductors, and when the electric spark is faint this medium can
be observed, for nitrogen will produce a blue tinge and carbonic acid
a green; hydrogen gives us a red, as already remarked. By multiplying
the number of eggs and plates of glass, and placing discs of tinfoil
in various shapes at certain distances, many beautiful figures may be
observed when the spark is set free.
Professor Tyndall at the Royal Institution showed a very pretty
experiment. He took a funnel with a very fine bore, and permitted
sand to flow from it as it will in the hour-glass. When he permitted
the electric current to come in contact with the sand, however, it,
instead of falling vertically to the table, spread out fan-like, each
grain repelling and being repelled by its neighbour with an effect very
beautiful to see. Luminous effects have frequently been produced by
passing an electric spark through various bodies. For instance, a lump
of sugar can be made quite brilliant in the dark by passing electricity
through it, and there are other substances similarly affected. Even
eggs and some fruits are thus made phosphorescent. The illumination of
the “diamond” covered Leyden jar is familiar to all who ever attended a
lecture on Electricity.
The various effects of the electric discharge need not here be
described. We have witnessed the results of lightning, but even in our
laboratory many pretty little experiments can be made, such as the
perforation of a card by the electric discharge. The chemical effects
are various. Decomposition of water is effected by electricity, and the
discharge can also be, and has been utilized for military purposes,
such as employed by Professor Abel in his fuse, and in his apparatus
for firing mines. Experiments at Chatham and elsewhere have been very
successful in the application of electricity to modern warfare.
We will illustrate one or two of these. A thick card should be placed,
as in the illustration (fig. 216), between two insulated points, and
to the lower portion of the apparatus a chain be attached, held in the
hand, and wound round the Leyden jar. If then the knob of the jar and
the knob above the upper point be brought together, the spark will pass
through the card.
[Illustration: Fig. 216.—Experiment with card.]
In the same manner a glass may be perforated if the current be
stronger. Of course the whole apparatus—and particularly the plate—must
be quite dry, and it will be better to put a drop of oil under the
upper needle point so as to prevent the electricity spreading over the
glass. The glass will be found pierced by the electric spark when the
Leyden jar is brought into requisition (fig. 217).
It was at one time contended that the sudden expansion of the air by
the electric spark caused heat to be generated, and a thermometer was
invented by Kinnersley to show this. The illustration (fig. 218) will
explain it. When the electric spark passed between the balls in the
large tube the water rose in the smaller. But the immediate return of
the water to its level showed that the disturbance was only mechanical,
and not owing to heat.
[Illustration: Fig. 217.—Experiment with glass.]
[Illustration: Fig. 218.—Kinnersley’s thermometer.]
We may now pass from the “frictional” to the other kind—viz.,
“dynamical” electricity, and we shall begin with the consideration of
GALVANI’S experiments and the VOLTAIC PILE.
[Illustration: Electric condenser.]
FOOTNOTES:
[14] GANOT: _Eléments de Physique_.