How can electricity set a house on fire?
THIS IS ONE OF THE MOST IMPORTANT SECTIONS IN THE BOOK.
Do you know that you can make an arc light with two ordinary pencils?
The next experiment, which should be done by the class with the help
of the teacher, shows how to do it.
EXPERIMENT 71. Sharpen two pencils. About halfway between the
point and the other end of each pencil cut a notch all the way
around and down to the "lead," or burn a notch down by means
of the glowing resistance wire. What you call the "lead" of
the pencil is really graphite, a form of carbon. The leads of
your two pencils are almost exactly like the carbons used in
arc lights, except, of course, that they are much smaller.
Turn off the electricity both at the snap switch and at the
knife switch. Fasten the bare end of a 2-foot piece of fine
insulated wire (about No. 24) around the center of the lead in
each pencil so that you get a good contact, as shown in Figure
132. Fasten the other bare end of each wire to either side
of the open knife switch so that when this switch is open the
electricity will have to pass down one wire to the lead of one
pencil, from that to the lead of the other pencil, and from
that back through the second wire to the other side of the
knife switch and on around the circuit, as shown in Figure
133. Keep the two pencils apart and off the desk, while some
one turns on the snap switch and the "flush" switch that lets
the electricity through the resistance wire. Now bring the
pencil points together for an instant, immediately drawing
them apart about half an inch. You should get a brilliant
white arc light.
_Caution: Do not look at this brilliant arc for more than a
fraction of a second unless you look through a piece of smoked
or colored glass._
Blow out the flame when the wood catches fire. After you have
done this two or three times, the inside of the wood below the
notches will be burned out so completely that you can pull it
off with your fingers, leaving the lead bare all the way up to
the wires.
Let the class stand well back and watch the teacher do the
next part of the experiment.
Connect two heavy insulated copper wires, about No. 12, to
the sides of the knife switch just as you connected the
fine wires. But this time bring the ends of the copper wires
themselves together for an instant, then draw them apart. Hold
the ends of the wires over the zinc of the table while you do
this, as melted copper will drop from them.
[Illustration: FIG. 133. The pencil points are touched together and
immediately drawn apart.]
[Illustration: FIG. 134. A brilliant arc light is the result.]
WHAT HAPPENS WHEN AN ARC IS FORMED. What happens when you form an
electric arc is this: As you draw the two ends of the pencils apart,
only a speck of the lead in each touches the other. The electricity
passing for an instant through the last speck at the end of the pencil
makes it so hot that it turns to vapor. The vapor will let electricity
go through it, and makes a bridge from one pencil point to the other.
But the vapor gets very hot, because it has a rather high resistance.
This heat vaporizes more carbon and makes more vapor for the
electricity to pass through, and so on. The electricity passing
through the carbon vapor makes it white hot, and that is what causes
the brilliant glow. Regular arc lights are made exactly like this
experimental one, except that the carbons used are much bigger and are
made to stand the heat better than the small carbons in your pencil.
Carbon is one of those substances that turn directly from a solid to
a gas without first melting. That is one reason why it is used for arc
lights. But copper melts when it becomes very hot, as you saw when you
made an arc light with the copper wires. So copper cannot be used for
practical arc lights.
FIRES CAUSED BY ARCS. There is one extremely important point about
this experiment with arcs: most fires that result from defective
wiring are caused by the forming of arcs. You see, if two wires touch
each other while the current is passing and then move apart a little,
an arc is formed. And you have seen how intensely hot such an arc is.
Two wires rubbing against each other, or a wire not screwed tightly to
its connection, can arc. A wire broken, but with its ends close enough
together to touch and then go apart, can cause an arc. And an arc is
very dangerous in a house if there is anything burnable near it.
Wires should never be just twisted together and then bound with tape
to form a joint. Twisted wires sometimes break and sometimes come
loose; then an arc forms, and the house catches fire. Good wiring
always means soldering every joint and screwing the ends of the wires
tightly into the switches or sockets to which they lead.
[Illustration: FIG. 135. An arc lamp. The carbons are much larger than
the carbons in the pencils, and the arc gives an intense light.]
KEEPING ARCS FROM FORMING. Well-wired houses have the wires brought
in through iron pipes, called _conduits_, and the conduits are always
grounded; so if an arc should form anywhere along the line, the house
would be protected by an iron conduit and if one of the loose ends of
wire came in contact with the conduit, the current would rush to the
ground through it, blowing out a fuse. The next section tells about
the purpose of fuses.
The directions that usually come with electric irons, toasters, and
stoves say that the connection should be broken by pulling out the
plug rather than by turning off the switch. This is because the switch
in the electric-light socket sometimes loses its spring and instead
of snapping all the way around and quickly leaving a big gap, it moves
only a little way around and an arc is formed in the socket; if you
hear a sizzling sound in a socket, you may be pretty sure that an arc
has been formed. But when you pull the plug entirely out of the iron
or stove, the gap is too big for an arc to form and you are perfectly
safe.
Fire commissions usually condemn extension lights, because if the
insulation wears out on a lamp cord so that the two wires can come
in contact, a dangerous arc may easily form. And the insulation might
suddenly be scraped off by something heavy moving across the cord.
This can happen whether the light at the end of the cord is turned on
or off. So it is best if you have an extension light always to turn it
off at the socket from which the cord leads, not at the lamp itself.
Many people do not do this, and go for years without having a fire.
But so might you live for years with a stick of dynamite in your
bureau drawer and never have an explosion. Still, it is not wise to
keep dynamite in your bureau.
Arc lights themselves, of course, are no more dangerous than is a
fire in a kitchen stove. For an arc light is placed in such a way
that nothing can well come near it to catch fire. The danger from
the electric arc is like the danger from gasoline spilled and
matches dropped where you are not expecting them, so that you are not
protected against them.
Fortunately ordinary batteries have not enough voltage to cause
dangerous arcs. So you do not have to be as careful in wiring for
electric bells and telegraph instruments. It requires the high voltage
of a city power line to make a dangerous electric arc.
So many fires are caused by electric arcs forming in buildings, that
you had better go back to the beginning of this section and read it
all through again carefully. It may save your home and even your life.
After you have reread this section, test your understanding of it by
answering the following questions:
1. How can you make an electric arc?
2. Why should wires not be twisted together to make electric
connections?
3. Why should wires be brought into houses and through walls in iron
conduits?
4. Why should you pull out the plug of an electric iron, percolator,
toaster, heater, or stove?
5. Why do fire commissions condemn extension lights?
6. If you use an extension light, where should it be turned off?
7. If you hear a sizzling and sputtering in your electric-light
socket, what does it mean? What should you do?
8. Is there any danger in defective sockets with switches that do not
snap off completely? What is the danger?
9. In Application 55, page 228, if the rat had gnawed the wire in two
while the electric iron was being used, would anything have happened
to the rat? Would there have been any danger to the house?
10. Where a wire is screwed into an electric-light socket, what harm,
if any, might result from not screwing it in tightly?
11. How can a wire be safely spliced?
12. Why is an electric arc in a circuit dangerous?
INFERENCE EXERCISE
Explain the following:
351. White objects look blue when seen through a blue glass.
352. When you pull the plug out of an electric iron, the iron
cools.
353. People who do not hear well sometimes use speaking
trumpets.
354. The sounding board of a piano is roughly triangular; the
longest strings are the extreme left, and those to the right
get shorter and shorter.
355. Birds can sit on live wires without getting a shock.
356. Deaf people can sometimes identify musical selections by
holding their hands on the piano.
357. An electric toaster gets hot when a current passes
through it.
358. The cord of an electric iron sometimes catches fire while
the iron is in use, especially if the cord is old.
359. If a live wire touches the earth or anything connected
with it, the current rushes into the earth.
360. When you stub your toe, you have to run forward to keep
from falling.