ELECTRIC LIGHT
MAN is not the only living being which sees by artificial
light. Cats’ eyes shine in the dark. So do glow-worms, and in South
America there are some insects which shed so bright a light that they
are used by the natives as lamps. These lights produced by animals are
much superior to the artificial lights which we can produce. They are
very economical. The animal expends very little of its strength in
producing the light, and the light is much cooler in proportion to its
brightness than any other light we know. Ever since man has learned how
to kindle a fire he has been able to light up the darkest night. He no
longer depends upon the sun and moon. But the fire which savages make
with logs of wood not only gives light, but also heat. It gives far
more heat than light. It is so hot that one cannot touch it, and yet
we can look into its blaze without feeling hurt by the glare. It could
be much brighter than it is without blinding us.
Lamps were invented probably by the ancient Egyptians. They consisted
of a shallow vessel full of oil, with a wick dipping into it. Candles
were used later, when it was found that certain kinds of fat could
be made to melt just where the wick touched them. But even the best
candles give a poor light, unless hundreds of them are lighted at the
same time, and then they give a great deal of smoke. The best light of
all would be a light which was very bright, but gave no heat, smoke,
or smell. It would have to be also very cheap, and so arranged that it
could be lighted at once at any time and in any place.
The last century has seen wonderful improvements made in lamps, and
further improvements are made every day. The most convenient light
invented up to the present time is the electric light. It gives no
smoke, it is clear and bright, it can be turned on with a slight
touch, and it is cheap. It is better than oil lamps, since there is
no trimming of wicks and no oil to fill up. It is better than gas,
since it requires no match for lighting it, and there is no danger of
explosion.
[Illustration: Fig. 63.--An Arc Lamp.]
The invention of the electric light belongs entirely to the nineteenth
century. Sir Humphry Davy was the first to produce it. He had a great
battery of two thousand cells at the Royal Institution, and he found
that when he joined the two ends of the battery by wires and separated
the wires again, a vivid light played between the two ends of the
wires. This light or flame was so hot that it melted all metals,
including platinum, which is extremely difficult to melt. He tried
to find some substance which would not melt, but would yet conduct
electricity. He tried charcoal, but found it conducted the current
very badly. He plunged it in mercury, and so obtained a fairly good
conductor, which did not melt in the electric flame, but burned away
rather quickly. However, he showed this light at the Royal Institution
so long ago as the year 1808, and it excited great wonder and curiosity.
For many years this remained the only experiment, and it was only
towards the middle of the century that a substance was found which
burned away very slowly. That substance was discovered at the bottom
of the crucibles used for making coal gas. When all the coal gas had
been driven out of the coal by heat, and only the coke remained, it was
found that a very hard and compact black mass remained at the bottom
of the crucible. That substance was called “gas carbon.” It conducted
electricity well, and made a very good substance for experiments on
the electric flame. It was cut into small rods, and the flame was made
to play between the ends of two such rods.
The flame was always seen to be in the form of an arch or arc, curving
upwards between the two rods. This was due to the heat of the flame
which produced an upward current of air between the pencils. The flame
was therefore called the “electric arc,” and a lamp containing it was
called an “arc lamp.” Such arc lamps are used to the present day for
lighting streets and squares. One such lamp is as bright as several
thousand candles burning together. The light which comes from the white
hot ends of the carbons is the brightest that it is possible to produce
artificially.
But the arc lamp is really too bright for ordinary use. It is quite
blinding, and it would not do to bring it into an ordinary room. For
many years electricians were busy trying to make another kind of
electric lamp which would not give such a glaring light as the electric
arc. The two men who did most to solve this problem were Thomas Edison
in the United States of America and Joseph Swan in England.
Edison was born in the year 1847. He never went to school. All the
education he ever got was given to him by his mother.
[Illustration: Fig. 64.--Thomas Edison.]
At the age of twelve he was put to work on a railway. Having a great
deal to do with newspapers, he managed to pick up the art of printing,
and by and by he printed a newspaper of his own in a luggage van. One
day he rescued a station-master’s child which was nearly run over by
a train. The station-master, out of gratitude, taught him how to work
the telegraph, which had been introduced just then. Young Edison very
soon acquired great skill at the telegraph, and was soon employed as a
telegraphist.
When only twenty-one Edison invented a new telegraph which could
print its own message. That invention was found extremely useful, and
several telegraph companies asked him to invent other things for them
which were wanted badly. Edison set up a factory of his own for making
printing telegraphs, but within a few years he sold his factory, which
had become too small, and set up a large new establishment at Menlo
Park, in the city of New Jersey. That establishment has since grown
to a great size, and Edison is still working in it at many wonderful
inventions.
Edison’s great idea was to use electricity to make life easy and
pleasant. In order to do this he proceeded in a rather different
manner from the earlier discoverers. In Europe the object of men of
science has usually been first to discover a new truth, and then to
find in what way the discovery could be made useful to humanity. Some
scientific men consider that the first object is much more important
than the second.
It is told of a certain professor who had made a great discovery in
chemistry, that his greatest satisfaction was the thought that it was
never to be of the slightest use to anybody. The story, of course, is
not true, or at least it is greatly exaggerated, for no great discovery
in science can be entirely useless. It enlarges our knowledge of the
nature and constitution of the world, increases our power over matter,
and with every increase of power we are better able to improve our
surroundings and make nature do our will.
Edison, however, proceeded in a different way. He first of all asked
himself what great problem wanted a solution. Having found the problem,
and stated it clearly, he collected all the facts and observations that
had any connection with the problem. Then he looked for the best way in
which to attempt the solution, and if necessary he made a vast number
of new experiments and investigations. In this way he not only made
great inventions, but great scientific discoveries as well.
When he set himself to find a convenient electric lamp, he knew that
when a strong current is sent through a thin wire, that wire becomes
red hot, or even white hot, and therefore gives a light. Such a light
is not a glaring one, and therefore is better in one respect than an
electric arc. Edison tried a great many different wires, and although
many of them gave a good light, they did not last long. A platinum wire
could be made white hot for some time, but in the end it always broke
in some place.
As the electric arc had been constructed successfully with carbon,
Edison naturally tried carbon next. Charcoal was found to be too
porous, and gas carbon could not be drawn into wires. It then struck
Edison that if a silk thread were subjected to great heat, it was
turned black. It would, so to speak, be made into carbon like charcoal,
and would not require thinning down, as it would be quite thin. He
tried the threads of silk, cotton, and flax, and heated them strongly
in a closed vessel. They produced very fine threads of carbon, but
these threads broke when they were in the least disturbed. Of all the
substances which Edison tried, a bamboo fibre was found to be the best,
but even that wore away after being used in a lamp for six hundred
hours.
Edison next invented what he called an “electric candle.” It was a
short rod made of platinum powder mixed with lime. But he shortly
afterwards came to the conclusion that nothing but a fine thread
resembling a wire, but made of carbon, would really answer the purpose.
It is strange that this conclusion was reached just about the same time
by Mr. Joseph Swan in England. The two inventors were working along
the same lines, and in the end they arrived at the same point. Edison
obtained good results by mixing lamp black and tar, and drawing it
into a fine thread. Joseph Swan, on the other hand, soaked a thread
of carbon in sulphuric acid, which made it into a kind of parchment.
He then blackened it by exposing it to great heat. Both inventors
were very successful, and produced lamps which were in every respect
satisfactory.
Both men had patented their inventions, and for some time there was
great rivalry between them, but in the end they decided that it was
better to be friends than enemies. They formed an alliance, and their
lamps were all made in the same way, and distinguished by the word
“Ediswan,” which is a combination of “Edison” and “Swan.” Cotton
wool was soaked in a solution of chloride of zinc. This gave a pasty
substance called cellulose. A lump of this was forced by pressure
through a very small hole, and so drawn out into a fine thread. The
thread was then blackened by heating it in a crucible made of plumbago,
and so the fibre was finished. The fibre was called a filament.
[Illustration: Fig. 65.--Electric Glow lamp with Carbon Filament.]
The next thing was to enclose the filament in a glass globe in which
not a trace of air would get near it. For if even the smallest quantity
of air got near the filament while it was glowing, it would at once
burn up. It was easy enough to pump all the air out of a glass globe,
and leave only some other gas in the globe that would do no harm, but
the difficulty was to mount a filament in the globe and to conduct a
current through it from the outside.
This was done by melting thin platinum wires into the glass. Platinum
wires were the only wires which would not break the glass. For when
platinum is heated it expands in the same way and to the same amount
as glass, and therefore it does not break away from it. The filament
was joined to the platinum wires by a special cement. After all this
had been done, the air was pumped out, and the globe was melted off
the pump at the lower end where the little point of glass is seen. The
glass globe is called a bulb. It is provided with a brass collar from
which two points stick out sideways. These points fit into recesses in
another brass collar into which the wires bearing the electric current
are led. This kind of joint is known as a bayonet joint.
The current for the lamps is furnished from a special factory in which
a steam-engine is made to turn an electric machine called a dynamo.
Such a dynamo is constructed on the principle of the induction currents
discovered by Faraday. Faraday’s whole life was spent in discovering
new truths. He did not think first of all of the use his discoveries
might be to mankind. He knew well that there were plenty of engineers
and inventors ready to take advantage of his discoveries. It is very
well that the discoverer and inventor should work hand in hand. The
discoverer shows the way in which great achievements can be attained.
The inventor makes these great practical achievements, and the good
that results from them comes back to the discoverer in the gratitude
of those who have been benefited by his discoveries and the happiness
which followed from them.