Switches and Fuses

An electric switch is often on a wall
near the door of a room. Two wires lead to the lamp in the room. The switch is
fixed in one of them. The switch can cause a break in this wire, and then the
light goes. The switch can also join the two parts of the wire again, then we
get a light.  

Switch can control many different
things. Small switches control lamps and radio sets because these do not take a
large current, larger switches control electric fires. Other switches can
control electric motors. 

 Good
switches move quickly. They have to stop the current suddenly. If they move
slowly, an electric spark appears. It jumps across the space between the two
ends of the wire. This is unsafe and it heats the switch. Very big switches are
sometimes placed in oil, Sparks do not easily jump through oil, and so the oil
makes the switch safer.

 A large current makes a wire hot.
If the wire is very thin, even a small current makes it hot. This happens in an
electric lamp. 

 The
electric wires in a house are covered with some kind of insulation. No current
can flow through the insulation, so the current can never flow straight from one
wire to the other, but the insulation on old wires is often broken, then the
copper of the two wires can touch. A large current may flow, and if this
happens, the wires will get very hot. Then the house may catch fire. 

 Fuses
can stop this trouble. A fuse is only a thin wire which is easily melted. It is
fixed in a fuse-holder(保险盒). The
fuse-holder is made of some material which cannot burn. A large current makes
the fuse hot and then it melts away. We say that the fuse "blows(<保险丝>烧断)".
The wire is broken and no current can flow. So the house does not catch fire,
but all the lights and electric fires go out because there is no current. 

 When a
fuse blows, something is wrong. We must find the fault first. Perhaps two wires
are touching. We must cover them with new insulation of some kind. Then we must
find the blown fuse and repair it. We put a new piece of fuse- wire in the
holder. (Sometimes we can find the right fuse- holder because it is rather warm,
but the others are cold.) If we do not repair the fault first, the new fuse will
blow immediately. 

 Some men
get angry when a fuse blow. So they put a thick copper wire in the fuse-holder!
Of course this does not easily melt; if the current rises suddenly, nothing
stops it. The thick wire easily carries it. Then the wires of the house may get
very hot and the house may catch fire. Some of the people in it may not be able
to escape. They may lose their lives. So it is always best to use proper
fuse-wire. This will keep everyone and everything in the house safe.