Why does a fuse blow out?
Sometimes during the evening when the lights are all on in your home,
some one tinkers with a part of the electric circuit or turns on an
electric heater or iron, and suddenly all the lights in that part of
the house go out. A fuse has blown out. If you have no extra fuses on
hand, it may be necessary to wait till the next day to replace the one
that is blown out. It is always a good idea to keep a couple of extra
fuses; they cost only 10 cents each. And if you do not happen to know
how fuses work or how to replace them when they blow out, it will cost
a dollar or so to get an electrician to put in a new fuse. The next
three experiments will help you to understand fuses.
[Illustration: FIG. 136. _A_, the "fuse gap" and _B_, the "nail
plug."]
EXPERIMENT 72. On the lower wire leading to the electric lamp
in the laboratory you will find a "gap," a place where the
wire ends in a piece of a knife switch, and then begins again
about an inch away in another piece of the switch, as shown in
Figure 136. There must be some kind of wire or metal that will
conduct electricity across this gap. But the gap is there to
prevent as much electricity from flowing through as might flow
through copper wire. So never put copper wire across this gap.
If you do, you will have to pay for the other fuses which may
blow out. Always keep a piece of fuse wire stretched across
the gap. Fuse wire is a soft leadlike wire, which melts as
soon as too much electricity passes through it.
Unscrew the lamp, and into the socket where it was, screw
the plug with the two nails sticking out of it. Turn the
electricity on. Does anything happen? Turn the electricity
off. Now touch the heads of the two nails together, or connect
them with a piece of any metal, and turn on the electricity.
What happens? Examine the pieces of the fuse wire that are
left.
It was so easy for the electricity to pass through the nails and wire,
that it gushed through at a tremendous rate. This melted the fuse
wire, or blew out the fuse. If the fuse across the gap by the socket
had not been the more easily burned out, one or perhaps both of the
more expensive fuses up above, where the wire comes in, would have
blown out. These cost about 10 cents each to replace, while the fuse
wire you burned out costs only a fraction of a cent. If there were
no fuses in the laboratory wirings and you had "short circuited" the
electricity (given it an easy enough path), it would have blown
out the much more expensive fuses where the electricity enters the
building. If there were no big fuses where the electricity enters
the building, the rush of electricity would make all the copper wires
through which it flowed inside the building so hot that they would
melt and set fire to the building. As long as you keep a piece of fuse
wire across the gap, there is no danger from short circuits.
WHY FUSE WIRE MELTS. For two reasons, the fuse wire melts when
ordinary wire would not. First, it has enough resistance to
electricity so that if many amperes (much current) flow through, it
gets heated. It has not nearly as much resistance, however, as the
filament in an electric lamp or even as has the long resistance wire.
It does not become white hot as they do.
Second, it has a low melting point. It melts immediately if you hold a
match to it; try this and see. Consequently, long before the fuse wire
becomes red hot, it melts in two. It has enough resistance to make it
hot as soon as too many amperes flow through; and it has such a low
melting point that as soon as it gets hot it melts in two, or blows
out. This breaks the circuit, of course, so that no more electricity
can flow. In this way the fuse protects houses from catching fire
through short circuits.
[Illustration: FIG. 137. What will happen when the pin is thrust
through the cords and the electricity turned on?]
Unfortunately, however, the fuse is almost no protection against an
electric arc. The copper vapor through which the electricity passes
in an arc has enough resistance to keep the amperage (current) low;
so the arc may not blow out the fuse at all. But if it were not for
fuses, there would be about as much danger of houses being set on
fire by short circuits as by arcs. Perhaps there would be more danger,
because short circuits are the more common.
EXPERIMENT 73. Put a new piece of fuse wire across the fuse
gap. Leave the "nail plug" screwed in the socket. Use a piece
of flexible lamp cord--the kind that is made of two strands of
wire twisted together (see Fig. 137). Fasten one bared end of
each wire around each nail of the "nail plug." See that the
other ends of the lamp cord are not touching each other.
Turn on the electricity. Does anything happen? Turn off the
electricity. Now put a pin straight through the middle of the
two wires. Turn on the electricity again. What happens?
There is not much resistance in the pin, and so it allows the
electricity to rush through it. People sometimes cause fuses to blow
out by pinning pictures to electric lamp wires or by pinning the wires
up out of the way.
A SHORT CIRCUIT AN "EASY CIRCUIT." You always get a short circuit when
you give electricity an easy way to get from one wire to the other.
But you get no current unless you give it some way to pass from one
wire to the other, thus completing the circuit. Therefore you should
always complete the circuit through something which resists the flow
of electricity, like an electric lamp, a heater, or an iron. Remember
this and you will have the key to an understanding of the practical
use of electricity.
The term "short circuit" is a little confusing, in that electricity
may have to go a longer way to be short circuited than to pass through
some resistance, such as a lamp. Really a short circuit should be
called an "easy circuit" or something like that, to indicate that it
is the path of least resistance. Wherever the electricity has a
chance to complete its circuit without going through any considerable
resistance, no matter how _far_ it goes, we have a short circuit. And
since everything resists electricity a little, a large enough flow
of electricity would even heat a _copper_ wire red hot; that is why a
short circuit would be dangerous if you had no fuses.
_APPLICATION 59._ To test your knowledge of short circuits and
fuses, trace the current carefully from the upper wire as it
enters the laboratory, through the plug fuse. Show where it
comes from to enter the plug fuse, exactly how it goes through
the fuse, where it comes out, and where it goes from there.
Trace it on through the cartridge fuse in the same way,
through all the switches into the lamp socket, through the
lamp, out of the lamp socket to the fuse gap, across this to
the other wire, and on out of the room.
It goes on from there through more fuses and back to the
dynamo from which the other wire comes.
Test yourself further with the following questions:
1. Where in this circuit is the resistance supposed to be?
2. What happens when you put a good conductor in place of this
resistance if the electricity can get from one wire to the other
without passing through this resistance?
3. Why do we use fuses?
4. What is a short circuit?
5. What makes an electric toaster get hot?
6. Why should you not stick pins through electric cords?
EXPERIMENT 74. Take the fuse wire out of the fuse gap and put
a single strand of zinc shaving in its place. Instead of the
nail plug, screw the lamp into the socket. Do not turn on the
switch that lets the electricity flow through the resistance
wire, but turn on the electricity so that the lamp will glow.
Does the zinc shaving work satisfactorily as a fuse wire?
Now turn the electricity on through the resistance wire. What
happens?
When are the greater number of amperes of electricity flowing
through the zinc shaving? (NOTE. "Amperes" means the amount of
current flowing.) Can the zinc shaving stand as many
amperes as the fuse wire you ordinarily use? Which lets more
electricity pass through it, the lamp or the resistance wire?
Why do electric irons and toasters often blow out fuses? If
this happens at your home, examine the fuse and see how many
amperes (how much current) it will allow to flow through it.
It will say _6A_ if it allows 6 amperes to pass through it;
_25A_ if it allows 25 amperes to pass through it, etc. The
fuse wire across the fuse gap allows about 8 amperes to pass
through before it melts. The zinc shaving allows only about
2. Read the marks on the cartridge and plug fuses. How many
amperes will they stand?
_APPLICATION 60._ A family had just secured an electric
heater. The first night it was used, the fuse blew out.
The boy said: "Let's put a piece of copper wire across the
fuse socket; then there can't be any more trouble."
The father said that they had better get a new fuse to replace
the old one. The old fuse was marked _10A_.
Was the boy or was the father right? If the father was right,
should they have got a fuse marked _6A_, one marked _10A_, or
one marked _15A_?
_APPLICATION 61._ The family were putting up an extension
light. They wanted the cord held firmly up out of the way.
One suggested that they drive a nail through both parts of the
cord and into the wall. Another thought it would be better to
put a loop of string around the cord and fasten the loop to
the wall. A third suggested the use of a double-pointed carpet
tack that would go across the wires, but not through them,
and if driven tightly into the wall would hold the wire more
firmly than would the loop.
Which way was best?
INFERENCE EXERCISE
Explain the following:
361. If the insulation wears off both wires of a lamp cord,
the fuse will blow out.
362. Street cars are heated by electricity.
363. The handles of pancake turners are often made of wood.
364. Glue soaks into the pores of pieces of wood and gradually
hardens.
365. The glue then holds the pieces tightly together.
366. You need a fuse of higher amperage, as a 10-ampere fuse,
instead of a 6-ampere one, where you use electricity for an
iron, and one of still higher amperage for an electric stove.
367. You should be careful about turning on electric lights or
doing anything with electric wires when you are on a cement,
iron, or earthen floor, or if you are standing in water.
368. The keys and buttons with which you turn on electric
lights are usually made of a rubber composition.
369. Defective wiring, because of which bare wires may touch,
has caused many fires.
370. A person wearing glasses can sometimes see in them the
image of a person behind him.