You have just learned that the light in an arc-lamp is caused by the
current forcing off from the carbon sticks tiny particles and heating
them up until they give a brilliant light. So, you see, in an arc-light
there is a wearing away of carbon by electricity, and therefore these
sticks, or pencils, of carbon in time are all burned away. In practice
the carbon pencils last about eight or ten hours, and then new ones
must be put in.
Now, in the incandescent lamp there is also carbon used, but the light
is not produced by the combustion or wasting away of the carbon, as we
will show you.
The picture below will show you the appearance of an incandescent lamp.
(Fig. 23.)
[Illustration: Fig. 23]
You will see that this lamp consists of a pear-shaped globe, and inside
is a long U-shaped strip of carbon no thicker than an ordinary thread.
This is a strip of bamboo cane[1] which has been carbonized to a thread
of charcoal. It is joined to two wires which come through the glass.
These two wires come down through the bottom of the globe, and one is
fastened to a brass screw-ring, while the other wire is fastened to a
brass button at the bottom of the lamp. These two (the ring and button)
must, as you know, be separated from each other by something which
will not carry electricity, or they would make a short circuit when the
electricity was applied. We separate the ring and the button in various
ways.
Now, if we took the ends of two wires which were charged with the
proper amount of electricity and put one wire on the screw-ring and the
other on the button, the lamp would light up, because there would be a
complete path for the current to travel in.
[Illustration: Fig. 24]
It will, however, be plain to you that it would be awkward to light the
lamps in this way, so we use a "socket" into which the lamp is screwed.
(Fig. 24.)
The wires from the dynamo carrying the electricity are connected in the
socket, one wire with the screw thread into which the screw-ring fits,
and the other with a button which the button on the lamp touches when
the lamp is screwed into the socket. Thus we have a connected path for
the current to travel in, or, as it is termed, a _complete circuit_.
You will notice that in the incandescent lamp the electricity does
not need to jump, as it does in the arc-light, because we give it one
continuous line to travel in.
In order, however, to get the current to do work for us, we put some
resistance in its path, which it must overcome in order to travel back
to the dynamo. The resistance in an incandescent lamp is the U-shaped
carbon strip (or, as it is called, "filament"). This charcoal filament
has so much greater resistance than the wires that it opposes, or
resists, the passage of the electricity through it; but the electricity
_must_ go through, and, as it is strong enough to force its way, it
overcomes this resistance and passes on through the carbon to the wire
at the other end. You see it is a struggle between the carbon and the
electricity, the current being determined to go on and the carbon
trying to keep it back; and, in the end, the electricity, being the
stronger, gets the best of it; but the struggle has been so hard that
the carbon has been raised to a white heat, or incandescence, and so
gives out a beautiful light, which continues as long as the current of
electricity flows.
You will remember that in the arc-light the carbons are slowly consumed
and new ones must be put in. If the carbon in the incandescent light
were consumed, it would not last many minutes, because it is only about
the size of a horsehair. Now, you will naturally inquire why this fine
strip is not burned up when it is raised to so high a heat. Well, we
will tell you.
You know that if you light a match and let it burn the wood will all be
consumed. But did you ever light a match, put it into a small bottle,
and put the cork in? If you never did, do so now as an experiment, and
you will see that the match will keep lighted for an instant and then
go out without consuming the wood.
The reasons for this are very simple. In order to burn anything up
entirely it is absolutely necessary to have the gas called oxygen
present, and, as the air you live in contains a very large amount of
oxygen, there is more than sufficient in your room to cause the wood
of the match to be entirely consumed after it is lighted. But there is
such a small quantity of oxygen in the bottle that it is not enough to
keep the fire going in the match, and, consequently, it will not burn
up the wood.
The reason the filament in an incandescent lamp is not burned up is
because there is _no oxygen_ inside the globe. After the carbon is
put in its place all the oxygen is drawn out through a tube, and the
glass is sealed up so that no more oxygen can get in. This is called
obtaining a "vacuum," and vacuum means a space without air.
There being no oxygen in the globe, it is impossible for the carbon to
burn up; so the incandescent lamp will continue to give its light for a
very long time, some of them lasting for thousands of hours. Some day,
however, from a great variety of obscure causes, the filament becomes
weak in some particular spot and breaks, and the light ceases. When
this happens, we unscrew the lamp and put another one in, and the light
goes on as usual.
Now you have learned how the incandescent lamp is made to give light.
We will add that it is a beautiful, soft, white light, almost without
heat, it will not explode, throws off no poisonous fumes like gas or
oil lamps, and has many other points of comfort and convenience which
make it very desirable.