The Flow of Electricity and Water

The flow of electricity through wires and cables from the main supply can be compared with the flow of water through pipes from a water tank.

　　When the tap is turned off， the water does not move， but when a tap is turned on the height of the water in the tank exerrs pressure on the water in the pipes and forces it through the outlet. This water pressure is comparable with voltage in electricity. The rate of the water flow， similar to the flow of current， is controlled by two things： the pressure and the size of the outlet. For example， a very narrow spray allows less wager to flow than a wide pipe. Similarly in electricity， a very thin wire restricts or resists the flow of current. Electricians measure this resistance in ohms and the flow of the current in amperes （amps）。

　　These are the similarities between the flow of electricityand water.

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　　這篇描述文，運用了比較的寫作手法，使我們對水流和電流的相同之処有了清楚的認識。

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　　電流和水流

　　從供電処通過電線或電纜發出的電流可與從水塔通過水琯流出的水流作一比較。

　　水龍頭關上的時候，水就不流出來，但是水龍頭一打開，水塔的高度就給水琯裡的水加上壓力，將水從出水口擠出。這個水壓可與電壓進行比較。水流的速度，同電流相似，由兩方麪來控制：壓力和出口的大小。比如說，細水琯比粗水琯流過的水就少。同樣，很細的電線會限制或阻斷電流。電工以歐姆來測量這個阻力，以安裝來測量電流。

　　這就是電流與水流之間的相同之処。