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开关和保险丝

zongxujian 于2008-05-24发布 l 已有人浏览
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  An electric switch

  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.

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