ELECTRIC SPARKS
WHEN the ships of the Carthaginians sailed round the coast
of Spain and France so far as Cornwall, to sell their purple cloth in
exchange for tin, they sometimes brought home with them small pieces of
a stone which had the colour of gold, but was transparent, and could be
cut more easily than glass. This stone came from the shores of Germany,
mostly from the Baltic Sea. It was not really a stone, but the hardened
sap of a sort of pine tree, which had lain long underground; and it
sometimes showed traces of forest life, such as bodies of insects,
embedded in it.
This stone, or “amber” as we call it now, was a source of wonder and
delight to the ancient Greeks and Romans. It was thought to possess
all kinds of magic properties. Besides looking very beautiful in a
necklace it was supposed to have a kind of soul of its own; for it
sometimes attracted little pieces of straw, or dust. The ancients could
only imagine that this was the effect of some kind of will or desire
possessed by the amber. They only knew of one other substance which
showed similar power. This was the lodestone which attracted iron, and
which the ancients therefore sometimes called “quick iron,” meaning
“living iron,” just as the lively liquid mercury was called quicksilver.
It was Dr. Gilbert of Colchester who first proved that these two powers
are entirely different from each other. He called the power shown by
amber “electric” power, and the power shown by lodestone he called
“magnetic” power. Electric power, he said, had to be excited by rubbing
the amber with a piece of cloth; but magnetic power could only be
excited in a piece of iron by rubbing it with lodestone, or hanging it
in a direction pointing to the North Pole. It was Dr. Gilbert also who
invented a little instrument resembling a compass which showed electric
attraction. It was a short piece of straw mounted on the point of a
needle in the same way as a magnetic needle.
Although Gilbert had proved that electric and magnetic forces are quite
different, an event occurred in the year 1681 which showed that there
is some kind of connection between the two. In that year a ship bound
for Boston was struck by lightning.
[Illustration: Fig. 46.--Gilbert’s Electric Needle.]
It was found by comparing the direction shown by the mariners’ compass
with that found by means of the North Star, that the north pole of the
compass pointed south, and the south pole north. In fact the needle had
been reversed, and the ship was steered into Boston harbour with the
compass reversed.
Even at that time people had guessed that the lightning had something
to do with electric force, a guess which Benjamin Franklin afterwards
proved to be true. Robert Boyle observed that hair easily shows
electric attraction. It can be “electrified,” as we say now. When a
cat has been lying by the fire for some time and we stroke her with
the hand, we hear a slight crackling noise as if from small sparks.
Some parts of the fur stick together, others stand apart. This is also
observed by ladies when they comb their hair. When the hair is very
dry, individual hairs fly apart.
False hair, which is usually drier than growing hair, shows electrical
attraction very strikingly. Boyle observed this more than two hundred
years ago. He wrote to a friend, “false locks of hair brought to a
certain degree of dryness are attracted by the flesh of some persons.
Of this I had proof in two beautiful ladies who wore them. For at some
time I observed that they could not keep them from flying to their
cheek and sticking there, though neither of them had any occasion
for paint, nor used it. One of them gave me leave to make a further
experiment, and asking her to hold her warm hand at a convenient
distance from one of those locks taken off and suspended in free air,
as soon as she did this the lower end of the lock which was free
attached itself to her hand.”
It was soon found that other substances besides amber and hair show
electric attraction. Isaac Newton hung a round piece of glass in a
brass ring fixed about an inch above a table. He rubbed the glass with
some “rough and raking stuff,” till some very small fragments of very
thin paper laid on the table beside the glass began to be attracted and
moved nimbly to and fro, leaping upon the glass and remaining there a
while, then leaping down and resting there, then leaping up and perhaps
down and up again. This experiment is repeated easily by rubbing a
stick of sealing wax with a woollen cloth, and holding it over small
bits of paper.
[Illustration: Fig. 47.--Guericke’s Electrical Machine made with a Ball
of Sulphur.]
Instead of rubbing amber or glass, Guericke rubbed a ball of sulphur
with his hands. He made this ball by filling a round bottle with
flowers of sulphur and melting the sulphur until it was quite liquid.
Then he let it cool until it was solid, and then he broke the bottle
and took out the sulphur ball. He attached the ball to a stick, and
when he had electrified it by rubbing, he used it to attract feathers,
which he succeeded in leading all round a room by this electric force
only.
About the year 1730, a pensioner of the name of Stephen Gray found
that electric force may be produced at one end of a wire by holding
an electrified body near the other. He expressed this fact by saying
that the wire “conducts electricity.” He found that all bodies may be
divided into conductors and non-conductors. He proved that the human
body is a conductor, and created a sensation by suspending a boy on a
network of silk and showing that he conducted electricity. Instead of
suspending the boy on a silk net, he found that it was sufficient to
let him stand on a cake of resin, which did not conduct electricity.
This experimental work with the electric force was taken up in France
with great interest. The French experimenters found out that when
a body is electrified strongly, sparks may be drawn from it. One
experimenter had himself suspended on a silken net, and got the others
to electrify him. When that was done sparks could be drawn out of his
body by any one who touched him. These sparks made a crackling noise
and produced a pricking sensation. “I shall never forget the surprise
caused by the first electric spark which was ever drawn from the human
body,” exclaimed one of those who were present.
So great was the interest roused by these new observations that people
began to make electric machines. One of the best of these was made by
Professor Bose in 1744. It consisted of a glass globe mounted in a
frame and kept rotating by means of a pulley.
[Illustration: Fig. 48.--Electricity produced by Friction with Glass
and conducted by a Metal Tube.]
One of the experimenters held his hands on the glass globe, and so
electrified it. The electricity was conducted by a long tin tube
suspended by silk threads. The end of the tube next the glass globe was
provided with linen threads, which collected the electricity into the
tube. At the other end stood an assistant on a cake of pitch or resin.
He held a sword in his hand, and from the end of that sword sparks
could be drawn as soon as the assistant touched the tin tube with his
other hand. These sparks could be made to set fire to spirits of wine
held in an iron dish by a servant who stood on the floor.
About the same time another German professor used a glass cylinder
instead of a glass globe, and did away with the necessity of rubbing
the glass with the hand by letting a leather cushion rub against it.
This leather cushion was a kind of artificial hand. But it was not
sufficiently like the human hand to be very useful, and was afterwards
improved by covering with a mixture of tin and mercury. Later still,
glass discs were used instead of globes or cylinders.
It was in the year 1745 that electric experiments became very popular,
owing to the accidental discovery of a piece of apparatus by which
the electric discharge, or spark, could be made much stronger. This
discovery was made about the same time in Pomerania and in Holland.
In both places people were trying to electrify water in a bottle. The
electricity was conducted into the water by a long nail, which stuck
out of the bottle at the neck. The bottle was held by an assistant,
and when enough electricity had been conducted into the water, the
assistant took the nail out of the water. In doing so he received a
great shock which paralysed his arms and shoulders. In Holland, this
experiment was performed at Leyden, and the jar which can be made to
give such a great shock is therefore called a “Leyden jar.”
The man who received the electric shock said that he would not take
another for the kingdom of France. But this feeling was not shared by
others, and many people tried the electric shock just to see what it
was like. Professor Bose went so far as to say he wished he might die
by the electric shock so that the account, of his death might furnish
an article for the French Academy of Sciences.
[Illustration: Fig. 49.--Discovery of the Leyden Jar.]
The new apparatus became so popular that a great number of people
made a living by going from place to place to exhibit it. In France
especially such entertainments attracted large crowds. On one occasion
an electric discharge was sent through a row of 180 soldiers. At
another time several hundred monks were made to join hands in a row 900
feet long, and a strong electric spark was sent through the whole row.
It made all the monks jump at the same moment.
It was not long before these experiments were repeated in America, and
when they did so they soon attracted the attention of a man who was
able to develop them much further. That man was Benjamin Franklin, who
was then engaged in the printing business. He saw the first experiment
in the year 1746, and at once began to investigate the subject. He was
the first to observe what a very important effect metallic points have
in drawing out, or throwing off, what the French called the “electric
fire.” He constructed a Leyden jar, and made many experiments with it.
When the hot summer weather came he proposed to his friends to wind up
his experiments with an electric dinner, for which a turkey was to be
killed by an electric shock and roasted in front of a fire lighted by
electricity.
In the year 1749, Franklin had an idea of making experiments to prove
that lightning was really a great electric spark. Before he saw the
electric experiments, he had thought that the lightning was due to
some kind of sulphurous breath coming from the earth and collecting
in the atmosphere. Other people thought that the lightning was due to
a gas explosion. The truth of such guesses could be decided only by
experiment. For when a thing has to be explained it is not sufficient
that it shall be more or less like some other thing. Things are
sometimes very similar and still have nothing to do with each other.
The flowers of frost on a pane of glass look very much like the leaves
of ferns or other delicate plants. But the force which makes the fern
grow in the wood is quite different from the frost which makes the
crystals grow on the glass, and it would be a great mistake to suppose
that the same causes are at work in both cases. For in one case we must
have a germ, nourishment, and a certain temperature, whereas in the
other case we require moisture and cold only.
Benjamin Franklin had the true scientific spirit. When he found that
the electric spark and lightning both looked bright and made a noise,
he did not jump at once to the conclusion that they had the same
nature. By careful study he found that lightning and the electric
spark have nearly the same colour. The electric spark looks like a
small flash of lightning. Both are equally swift, both are conducted
by metals, both are capable of tearing or breaking bodies they pass
through, both are capable of killing animals instantly, both can melt
metals and fire inflammable substances, and finally both give rise to
the same kind of smell.
It only remained now for Franklin to draw an electric spark from the
clouds to prove that lightning was an electric discharge. To do this
he proposed that a pointed iron rod should be mounted on the top of
some high steeple as near as possible to the clouds, so as to draw
electricity out of them. Near the top of the tower a man was to be
placed on a stool with glass legs, and connected with a pointed rod. If
the rod collected electricity from the clouds it would be possible to
draw sparks from the man. Franklin proposed this experiment in a letter
to the Royal Society of London, but the Society did not encourage him.
He then tried to start a lottery in order to obtain the money necessary
to build a tower for the purpose, since there was no building in
Philadelphia high enough for the experiment.
While Franklin was still making plans, he heard that the experiment
had been performed successfully near Paris. A man named Dalibard had
mounted a metal rod forty feet high on the roof of a small cabin and
rested the foot of the rod on a table inside the cabin. Dalibard set an
old dragoon to watch for thunder clouds, and had a wire ready near the
rod to draw off any sparks that might be obtained. After several days a
thunder cloud appeared, and sparks flew from the rod to the wire. The
dragoon thought they smelt of sulphur and that some demon was at work.
He sent for the priest, but the priest knew that this was not the work
of a demon; and he proceeded to draw sparks out of the rod with the
wire, after which he told Dalibard what had happened. The latter was
delighted, and exclaimed that Franklin’s idea was no longer a guess but
a reality.
Franklin, however, did not regard the experiment as a sufficient
proof, and he conceived the new idea of sending up a kite with an iron
point in order to draw electricity from the clouds. He sent up a kite,
holding the string by a silk ribbon, so as not to get the sparks into
his hand. A thunder storm came on, but no effect was observed although
Franklin had tied a key to the end of the string. But when the rain
began to fall and the string got wet, a strong spark was obtained on
bringing the knuckle near the key. This conclusive experiment had a
great effect all through the civilised world. It was repeated in every
country, and Franklin’s idea of drawing electricity out of the clouds
by means of pointed rods connected with the earth was adopted in many
places. Such rods were called Lightning Conductors.
Sometimes these lightning conductors were not connected with the earth,
but with some apparatus for measuring the amount of electricity, or
with a Leyden jar for collecting it. Such an arrangement was put up
in St Petersburg by a man of the name of Richmann. But while he and a
friend were observing the measuring instrument, the friend suddenly
saw a ball of blue fire come out of the rod and hit Richmann on the
head, killing him instantly. This death of an experimenter did not
excite horror, but rather envy. One great discoverer is said to have
exclaimed that it was not given to every electrician to die in so
glorious a manner. Men of science were interested greatly in the manner
in which the electric shock had affected the head and body of Richmann,
and those effects were investigated closely. Richmann himself was so
devoted to the cause of science that we may well believe that he would
have been proud of being the first to be killed in an experiment of
this kind. Besides, he was also the last, as his successors were able
through his death to protect themselves from similar dangers.