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.
