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    Electricity

    The modern age is an age of electricity. People are so used to electric lights， radio， televisions， and telephones that it is hard to imagine what life would be like without them. When there is a power failure， people grope about in flickering candlelight， cars hesitate in the streets because there are no traffic lights to guide them， and food spoils in silent refrigerators. Yet， people began to understand how electricity works only a little more than two centuries ago. Nature has apparently been experimenting in this field for millions of years. Scientists are discovering more and more that the living world may hold many interesting secrets of electricity that could benefit humanity. All living cells send out tiny pulses of electricity. As the heart beats， it sends out pulses of record；  they form an electrocardiogram， which  a doctor can study  to determine  how well the heart is working. The brain， too， sends out brain waves of electricity， which can be recorded in an electroencephalogram.  The electric currents  generated by most living cells are  extremely small —— often so small that sensitive instruments are needed to record them. But in some animals， certain muscle cells have become so specialized as electrical generators that they do not work as muscle cells  at  all. When  large  numbers of  these cells are linked  together， the  effects can  be astonishing. The electric eel is an amazing storage battery. It can send a jolt of as much as eight hundred volts of electricity through  the water in  which  it  lives. （An electric house current is only one hundred twenty volts.）  As  many as four-fifths  of all  the  cells in  the electric eel's  body are specialized for generating electricity， and the strength of the shock it  can deliver corresponds roughly to the length of its body.電當(dāng)今時(shí)代是電氣時(shí)代。 人們對(duì)電燈、收音機(jī)、電視和電話早已司空見(jiàn)慣以致很難想 象沒(méi)有它們生活會(huì)變成什么樣。

    當(dāng)停電時(shí)，人們?cè)趽u曳不定的燭光下暗中摸索； 因沒(méi)有紅 綠燈的指示，汽車(chē)在道路上遲疑不前；冰箱也停止工作，導(dǎo)致食物變質(zhì)。人們只是在兩個(gè)世 紀(jì)前一點(diǎn)才開(kāi)始了解電的使用原理，自然界卻顯然在這方面經(jīng)歷過(guò)了數(shù)百萬(wàn)年。  科學(xué)家不 斷發(fā)現(xiàn)許多生物世界里可能有益于人類的關(guān)于電的有趣秘密。所有生物細(xì)胞都會(huì)發(fā)出微小的 電脈沖。

    當(dāng)心臟跳動(dòng)時(shí)，把它發(fā)出的脈沖記錄下來(lái)就成了心電圖，這可讓醫(yī)生了解心臟的 工作狀況。大腦也發(fā)出腦電波，這可在腦電圖上記錄下來(lái)。

    許多生物細(xì)胞發(fā)出的電流都是 極微小的，小到要用靈敏儀器才能記錄和測(cè)量。  但一些動(dòng)物的某些肌肉細(xì)胞能轉(zhuǎn)化成一個(gè) 個(gè)發(fā)電機(jī)，以致完全失去肌肉細(xì)胞的功能。

    這種細(xì)胞大量地連接在一起時(shí)產(chǎn)生的效果將是 非常令人吃驚的。電鰻就是一種令人驚異的蓄電池。  它可以在水中發(fā)出相當(dāng)于 800 伏特電 壓電流（家庭用戶的電壓只有  120  伏特）。  在電鰻的身體里，多至五分之四的細(xì)胞都專門(mén)用 來(lái)發(fā)電，而且發(fā)出的電流的強(qiáng)度大約和它身體的長(zhǎng)度成正比。