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thickness be diminished, or the quantity of fluid passing over it
be augmented, or, in general, if the ratio of the fluid to the
magnitude of the space afforded to it be increased, the con-
ductor will be found to undergo an elevation of temperature,
which will be greater the greater the quantity of the electricity
and the less the space supplied for its passage.



250 ELECTRICITY.

1800. Experimental verification. Wire heated, fused, and
burned. If a piece of wire of several inches in length be
placed upon the stage of the universal discharger (1744), a feeble
charge transmitted through it will sensibly raise its tempera-
ture. By increasing the strength of the charge, its temperature
may be elevated to higher and higher points of the thermo-
metric scale ; it may be rendered incandescent, fused, vaporized,
and, in fine, burned.

With the powerful machine of the Taylerian Museum at
Haarlem, Van Marum fused pieces of wire above 70 feet in
length.

Wire may be fused in water ; but the length which can be
melted in this way is always less than in air, because the liquid
robs the metal of its heat more rapidly than air.

A narrow ribbon of tinfoil, from 4 to 6 inches in length,
may be volatilized by the discharge of a common battery. The
metallic vapour is in this case oxidized in the air, and its fila-
ments float like those of a cobweb.

1801. Thermal effects are greater as the conducting power
is less. These thermal effects are manifested in different
degrees in different metals, according to their varying conduct-
ing powers. The worst conductors of electricity, such as pla-
tinum and iron, suffer much greater changes of temperature by
the same charge than the best conductors, such as gold and
copper. The charge of electricity, which only elevates the
temperature of one conductor, will sometimes render another
incandescent, and will volatilize a third.

1802. Ignition of metals. If a fine silver wire be extended
between the rods of the universal discharger (1744), a strong
charge will make it burn with a greenish flame. It will pass
off in a greyish smoke. Other metals may be similarly ignited,
each producing a flame of a peculiar colour. If the experi-
ments be made in a receiver, the products of the combustion
being collected, will prove to be the metallic oxides.

If a gilt thread of silk be extended between the rods of the
discharger, the electricity will volatilize or burn the gilding,
without affecting the silk. The effect is too rapid to allow the
time necessary for the heat to affect the silk.

A strip of gold or silver leaf placed between the leaves of
paper, being extended between the rods of the discharger, will
be burnt by a discharge from ajar having two square feet of



THERMAL EFFECTS OF ELECTRICITY. 251

coating. The metallic oxide will in this case appear on the
paper as a patch of purple colour in the case of gold, and of
grey colour in that of silver.

A spark from the prime conductor of the great Haarlem
machine burnt a strip of gold leaf twenty inches long by an inch
and a half broad.

1803. Effect on fulminating silver. The heat developed in
the passage of electricity through combustible or explosive
substances, which are imperfect conductors, causes their com-
bustion or explosion.

A small quantity of fulminating silver placed on the point of
a knife, explodes if brought within a few feet of the conductor
of an electrical machine in operation. In this case the explo-
sion is produced by induction.

1804. Electric pistol. The electrical pistol or cannon is
charged with a mixture of hydrogen and oxygen gases, in the
proportion necessary to form water. A conducting wire ter->
minated by a knob is inserted in the touch-hole, and the gases
are confined in the barrel by the bullet. An electric spark
imparted to the ball at the touch-hole, causes the explosion of
the gases. This explosion is produced by the sudden combi-
nation of the gases, and their conversion into water, which, in
consequence of the great quantity of heat developed, is instantly
converted into steam of great elasticity, which, by its expansion,
forces the bullet from the barrel in the same manner as do the
gases which result from the explosion of gunpowder.

1805. Ether and alcohol ignited. Ether or alcohol may be
fired by passing through it an electric discharge. Let cold
water be poured into a wine-glass, and let a thin stratum of
ether be carefully poured upon it. The ether being lighter will
float on the water. Let a wire or chain connected with the
prime conductor of the machine be immersed in the water, and,
while the machine is in action, present a metallic ball to the
surface of the ether. The electric charge will pass from the
water through the ether to the ball, and will ignite the ether.
Or, if a person standing on an insulating stool, and holding in
one hand a metal spoon filled with ether, present the surface of
the ether to a conductor, and at the same time apply the other
hand to the prime conductor of a machine in operation, the
electricity will pass from the prime conductor through the
body of the person to the spoon, and from the spoon through



252 ELECTRICITY.

the ether, to the conductor to which the ether is presented, and
in so passing will ignite the ether.

1806. Resinous powder burned The electric charge trans-
mitted through fine resinous powder, such as that of colophony,
will ignite it. This experiment may be performed either
by spreading the powder on the stage of the discharger (1744),
or by impregnating a hank of cotton with it ; or, in a still more
striking manner, by sprinkling it on the surface of water con-
tained in an earthenware saucer.

1807. Gunpowder exploded Gunpowder may, in like

manner, be ignited by electricity. This experiment is most
conveniently exhibited by placing the powder in a small wooden
cup, and conducting the electric charge along a moist thread,
six or seven inches long, attached to the arm of the discharger,
which is connected with the negative coating of a jar, and
the charge, in its passage from one rod of the discharger to the
other, will ignite the powder.

1808. Electric mortars. The electric mortar, fig. 521., is

an apparatus by which the gunpowder is ignited
by passing an electric charge through it. The
mixed gases may also be used in this instru-
Fig. 521. ment .

Common air or gas, not being explosive, is heated so suddenly
and intensely by transmitting through it an electric charge,
that it will expand so as to project the ball from
the mortar.

1809. Kinnersley's electrometer. Kinnersley's
electric thermometer,^. 522., is an instrument in-
tended to measure the degree of heat developed in
the passage of an electric charge by the expansion
of air. The discharge takes place between the two
balls bb' in the glass cylinder, and the air confined
in the cylinder being heated expands, presses upon
the liquid contained in the lower part of the cy-
linder, and causes the liquid in the tube tf to rise.
The variation of the column of liquid in the tube
ft' indicates the elevation of temperature.



LUMINOUS EFFECTS OF ELECTRICITY. 253
CHAP. XL

LUMINOUS EFFECTS OF ELECTRICITY.

1810. Electric fluid not luminous. The electric fluid is not
luminous. An insulated conductor, or a Ley den jar or battery,
however strongly charged, is never luminous so long as the
electric equilibrium is maintained and the fluid continues in
repose. But if this equilibrium be disturbed, and the fluid
move from one conductor to another, such motion is, under
certain conditions, attended with luminous phenomena.

1811. Conditions under which light is developed by an elec-
tric current. One of the conditions necessary to the develop-
ment of light by the motion of the electric fluid is, that the
electricity should have a certain intensity. If the conductor of
an ordinary electric machine while in operation be connected
with the ground by a thick metallic wire, the current of the
fluid which flows along the wire to the ground will not be
sensibly luminous ; but if the machine be one of great power,
such for example as the Taylerian machine of Haarlem, an
iron wire of 60 or 70 feet long communicating with the ground
and conducting the current will be surrounded by a brilliant
light. The intensity of the electricity necessary to produce
this effect depends altogether on the properties of the medium
in which the fluid moves. Sometimes electricity of feeble in-
tensity produces a strong luminous effect, while in other cases
electricity of the greatest intensity developes no sensible de-
gree of light.

It has been already explained that the electric fluid with
which an insulated conductor is charged is retained upon it
only by the pressure of the surrounding air. According as
this pressure is increased or diminished, the force necessary to
enable the electricity to escape will be increased or diminished,
and in the same proportion.

When a conductor A in communication with the ground ap-
proaches an insulated conductor B charged with electricity,
the natural electricity of B will be decomposed, the fluid of the
same name as that which charges A escaping to the earth, and
the fluid of the opposite name accumulating on the side of B



254 ELECTRICITY.

next to A. At the same time, according to what has been ex-
plained (1785), the fluid on A accumulates on the side nearest
to B. These two tides of electricity of opposite kinds exert a
reciprocal attraction, and nothing prevents them from rushing
together and coalescing, except the pressure of the intervening
air. They will coalesce, therefore, so soon as their mutual
attraction is so much increased as to exceed the pressure of
the air.

This increase of mutual attraction may be produced by
several causes. First, by increasing the charge of electricity
upon the conductor A, for the pressure of the fluid will be pro-
portional to its depth or density. Secondly, by diminishing the
distance between A and B, for the attraction increases in the
same ratio as the square of that distance is diminished ; and
thirdly, by increasing the conducting power of either or both
of the bodies A and B, for by that means the electric fluids, being
more free to move upon them, will accumulate in greater quan-
tity on the sides of A and B which are presented towards each
other. Fourthly, by the form of the bodies A and B, for ac-
cording to what has been already explained (1776), the fluids
will accumulate on the sides presented to each other in greater
or less quantity, according as the form of those sides approaches
to that of an edge, a corner, or a point.

When the force excited by the fluids surpasses the restrain-
ing force of the intervening air, they force their passage through
the air, and, rushing towards each other, combine. This move-
ment is attended with light and sound. A light appears to be
produced between the points of the two bodies A and B, which
has been called the electric spark, and this luminous phenomenon
is accompanied by a sharp sound like the crack of a whip.

1812. The electric spark. The luminous phenomenon called
the electric spark does not consist, as the name would imply, of
a luminous point which moves from the one body to the other.
Strictly speaking, the light manifests no progressive motion.
It consists of a thread of light,
which for an instant seems to con-
nect the two bodies, and in gene-
ral is not extended between them
in one straight unbroken direction
like a thread which might be




LUMINOUS EFFECTS OF ELECTRICITY. 255

stretched tight between them, but has a zig-zag form resembling
more or less the appearance of lightning,^. 523.

1813. Electric aigrette. If the part of either of the bodies
A or B which is presented to the other have the form of a
point, the electric fluid will escape, not in the form of a spark,
but as an aigrette or brush light, the diverging rays of which
sometimes have the length of two or three inches. A very
feeble charge is sufficient to cause the .escape of the fluid when
the body has this form (1776).

1814. The length of the spark. If the knuckle of the finger
or a metallic ball at the end of a rod held in the hand be pre-
sented to the prime conductor of a machine in operation, a spark
will be produced, the length of which will vary with the power
of the machine.

By the length of the spark must be understood the greatest
distance at which the spark can be transmitted.

A very powerful machine will so charge its prime conductor
that sparks may be taken from it at the distance of 30 inches.

1815. Discontinuous conductors produce luminous effects.
Since the passage of the electricity produces light wherever the
metallic continuity, or more generally wherever the continuity
of the conducting material is interrupted, these luminous effects
may be multiplied by so arranging the conductors that there
shall be interruptions of continuity arranged in any regular or
desired manner.

1816. Various experimental illustrations. If a number of
metallic beads be strung upon a thread of silk, each bead being
separated from the adjacent one by a knot on the silk so as to
break the contact, a current of electricity sent through them will
produce a series of sparks, a separate spark being produced be-
tween every two successive beads. By placing one end of such
a string of beads in contact with the conductor of the machine,
and the other end in metallic communication with the ground,
a chain of sparks can be maintained so long as the machine is
worked.

The string of beads may be disposed so as to form a variety
of fancy designs, which will appear in the dark in characters of
light.

Similar effects may be produced by attaching bits of metallic
foil to glass. Sparkling tubes and plates are contrived in this




256 ELECTRICITY.

manner, by which amusing expe-
riments are exhibited. A glass
plate is represented \njig. 524. by
which a word is made to appear
in letters of light in a dark room.
The letters are formed by attaching

lozenge-shaped bits of tinfoil to the glass, disposed in the proper
form. In the same manner designs may be formed on the inner
surface of glass tubes, or, in fine, of glass vessels of any form.

In these cases the luminous characters may be made to ap-
pear in lights of various colours, by using spangles of different
metals, since the colour of the spark varies with the metal.

1817. Effect of rarefied air, When the electric fluid passes
through air, the brilliancy and colour of the light evolved de-
pends on the density of the air. In rarefied air the light is
more diffused and less intense, and acquires a reddish or violet
colour. Its colour, however, is affected, as has been just stated,
by the nature of the conductors between which the current
flows. When it issues from gold the light is green, from silver
red, from tin or zinc white, from water deep yellow inclining
to orange.

It is evident that these phenomena supply the means of pro-
ducing electrical apparatus by which an infi-
nite variety of beautiful and striking luminous
effects may be produced.

When the electricity escapes from a metallic
point in the dark, it forms an aigrette, ./fy. 525.,
which will continue to be visible so long as the
machine is worked.

The luminous effect of electricity in rarefied
air is exhibited by an apparatus,^. 526., consisting of a glass





Fig. 526.



receiver bb', which can be screwed upon the plate of an air-
pump and partially exhausted. The electric current passes



LUMINOUS EFFECTS OF ELECTRICITY. 257

between two metallic balls attached to rods, winch slide in air-
tight collars in the covers of the receiver bb'.

It is observed that the aigrettes formed by the negative fluid
are never as long or as divergent as those formed by the posi-
tive fluid, an effect which is worthy of attention as indicating a
distinctive character of the two fluids.

1818. Experimental imitation of the auroral light. This
phenomenon may be exhibited in a still more remarkable
manner by using, instead of the receiver bb', a glass tube two or
three inches in diameter, and about thirty inches in length. In
this case a pointed wire being fixed to the interior of each of the
caps, one is screwed upon the plate of the air-pump, while the
external knob of the other is connected by a metallic chain
with the prime conductor of the electrical machine. When the
machine is worked in the dark, a succession of luminous phe-
nomena will be produced in the tube, which bear so close a
resemblance to the aurora borealis as to suggest the most pro-
bable origin of that meteor. When the exhaustion of the tube
is nearly perfect, the whole length of the tube will exhibit a
violet red light. If a small quantity of air be admitted, luminous
flashes will be seen to issue from the two points attached to the
caps. As more and more air is admitted, the flashes of light
which glide in a serpentine form down the interior of the tube
will become more thin and white, until at last the electricity
will cease to be diffused through the column of air, and will ap-
pear as a glimmering light at the two points.

1819. Phosphorescent effect of the spark. The electric
spark leaves upon certain imperfect conductors a trace which
continues to be luminous for several seconds, and sometimes
even so long as a minute after the discharge of the spark.
The colour of this species of phosphorescence varies with the
substances on which it is produced. Thus white chalk pro-
duces an orange light. With rock crystal the light first red
turns afterwards white. Sulphate of barytes, amber, and loaf
sugar render the light green, and calcined oyster-shell gives all
the prismatic colours.

1820. Leichtenberg's figures. The spark in many cases
produces effects which not only confirm the hypothesis of two
fluids, but indicate a specific difference between them. One of
these has been already noticed. The experiment known as



258 ELECTRICITY.

Leichtenberg's figures presents another example of this. Let
two Leyden jars be charged, one with positive, the other with
negative electricity; and let sparks be given by their knobs to
the smooth and well-dried surface of a cake of resin. Let the
surface of the resin be then slightly sprinkled with powder of
Semen lycopodii, or flowers of sulphur, and let the powder thus
sprinkled be blown off. A part will remain attached to the
spots where the electric sparks were imparted. At the spot
which .received the positive spark, the adhering powder will
have the form of a radiating star ; and at the point of the nega-
tive spark it will have that of a roundish clouded spot.

1821. Experiments indicating specific differences between the
two fluids. If lines and figures be traced in like manner on
the cake of resin, some with the positive and some with the
negative knob, and a powder formed of a mixture of sulphur
and minium be first sprinkled over the cake and then blown
off, the adhering powder will mark the traces of the two fluids
imparted by the knobs, the traces of the positive fluid being
yellow, and those of the negative red. In this case the sul-
phur is attracted by the positive electricity, and is therefore
itself negative ; and the minium by the negative electricity, and
is therefore itself positive. The mechanical effects of the two
fluids are also different, the sulphur powder being arranged in
divergent lines, and the minium in more rounded and even
traces.

Let two Leyden jars, one charged with positive and the
other with negative electricity, be placed upon a plate of glass
coated at its under surface with tinfoil at a distance of six or
eight inches asunder, and let the surface of the. glass between
them be sprinkled with semen lycopodii. Let the jars be then
moved towards each other, and let their inner coatings be
connected by a discharging rod applied to their knobs. A
spark will pass between their outer coatings through the
powder, which it will scatter on its passage. The path of the
positive fluid will be distinguishable from that of the negative
fluid, as before explained, by the peculiar arrangement of the
powder ; and this difference will disappear near the point where
the two fluids meet, where a large round speck is sometimes
seen bounded by neither of the arrangements which charac-
terize the respective fluids.

1822. Electric light above the barometric column. The



LUMINOUS EFFECTS OF ELECTRICITY.



259





Fig. 5-27.



electric light is developed in every form of elastic fluid and
vapour when its density is very inconsiderable.
A remarkable example of this is presented in
the common barometer. When the mercurial
column is agitated so as to oscillate in the
tube, the space in the tube above the column
becomes luminous, and is visibly so in the
dark. This phenomenon is caused by the
effect of the electricity developed by the fric-
tion of the mercury and the glass upon the
atmosphere of mercurial vapour which fills the
space above the column in the tube.

1823. Cavendish's electric barometer. The
electric barometer of Cavendish, Jig. 527., il-
lustrates this in a striking manner. Two ba-
rometers are connected at the top by a curved
tube, so that the spaces above the two columns
communicate with each other. When the in-
strument is agitated so as to make the columns oscillate,
electric light appears in the curved tube.

1824. Luminous effects produced by imperfect conductors.
The electric spark or charge transmitted by means of the
universal discharger and Leyden jar or battery through various
imperfect conductors, produces luminous effects which are
amusing and instructive.

Place a small melon, citron, apple, or any similar fruit on the
stand of the discharger ; arrange the wires so that their ends
are not far asunder, and at the moment when the jar is dis-
charged the fruit becomes transparent and luminous. One or
more eggs may be treated in the same manner if a small wooden
ledge be so contrived that their ends may just touch, and the
spark can be sent through them all. Send a charge through a
lump of pipe-clay, a stick of brimstone, or a glass of water, or
any coloured liquid, and the entire mass of the substance will
for a short time be rendered luminous. As the phosphorescent
appearance induced is by no means powerful, it will be neces-
sary that these experiments should be performed in a dark
room, and indeed the effect of the other luminous electrical
phenomena will be heightened by darkening the room.

1825. Attempt to explain electric light, the thermal hypo-
thesis. No explanation of the physical cause of the electric



260 ELECTRICITY.

spark, or of the luminous effects of electricity, has yet been
proposed which has commanded general assent. It appears
certain, for the reasons already stated, and from a great variety
of phenomena, that the electric fluids themselves are not lu-
minous. The light, therefore, which attends their motion must
be attributed to the media, or the bodies through which or
between which the fluids move. Since it is certain that the
passage of the fluids through a medium developes heat in
greater or less quantity in such medium, and since heat, when
it attains a certain point, necessarily developes light, the most
obvious explanation of the manifestation of light was to ascribe
it to a momentary and extreme elevation of temperature, by
which that part of the medium, or the body traversed by the
fluid, becomes incandescent.

According to this hypothesis, the electric spark and the flash
of lightning are nothing more than the particles of air, through
which the electricity passes, rendered luminous by intense heat.
There is nothing in this incompatible with physical analogies.
Flame we know to be gas rendered luminous by the ardent heat
developed in the chemical combinations of which combustion is
the effect.

1826. Hypothesis of decomposition and recomposition.
According to another hypothesis, first advanced by Hitter and
afterwards adopted by Berzelius, Oersted, and Sir H. Davy,
the electric fluids have strictly speaking no motion of transla-
tion whatever, and never in fact desert the elementary mole-
cules of matter of which, according to the spirit of this hypo-
thesis, they form an essential part. Each molecule or atom
composing a body is supposed to be primitively invested with
an atmosphere of electric fluid, positive or negative, as the case
may be, which never leaves it. Bodies are accordingly classed
as electro-positive or electro-negative, according to the fluid
attracted to their atoms. Those atoms which are positive



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 26 of 45)