物理學 1800

論僅由不同種類導體相互接觸所激發的電

亞歷山卓·伏打

把兩種金屬夾著鹽水層層疊起，電第一次源源不斷地流。

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In depth · the introduction

兩千年來，電一直是一種你造得出、卻留不住的火花。伏打找到了讓它流動——並且持續流動——的辦法。

核心想法

拿兩種不同的金屬，再拿一塊浸了鹽水的布。每種金屬與濕布相接之處，都會冒出一點點推動電荷的「力」。單獨一個這樣的「三明治」，那點力小得察覺不到。可一旦把這些三明治朝同一方向疊起來——鋅、銀、濕布，鋅、銀、濕布——一點點的推力便會相加。疊上三四十個，你就有了一束穩定的電流，強到足以觸到、足以打出火花、足以做功。伏打，就此造出了第一塊電池。

成就它的那場爭吵

這一切，從一隻死青蛙開始。1780 年代，解剖學家路易吉·伽伐尼發現，蛙腿被兩種不同的金屬碰到時會抽動，於是斷定動物自帶一種「動物電」。科莫的物理學家伏打先是著迷，繼而起疑：他懷疑真正的源頭是那一對金屬，而非青蛙。

為了證明這一點，伏打乾脆把動物整個拿掉。他用濕紙板替下青蛙，把金屬圓片層層疊起，直到電堆本身就給出了那記從來不需要青蛙的電擊。1800 年 3 月，他把這一切寫成一封信，寄給倫敦皇家學會會長約瑟夫·班克斯爵士。電池，正是從兩個固執而出色的人長達十年的爭論中誕生的——而結果是，兩人都對了一半。

它為何重要

能持續的電流，才是你用得上的電流。短短數週之內，別人就用伏打的電堆把水拆成氫和氧，不久又用它撬開「元素」、發現新的元素——化學從此有了電這把手術刀。此後的每一塊電池，從手電筒到電動汽車，都是同一個想法：電池排成一列，電壓逐個相加。我們衡量這股推力的單位「伏特」，也以他命名。

像一段樓梯

把一個電池想成一級很矮的臺階——單獨一級，無足輕重。電堆則是一段樓梯：每一級都把電荷往上抬一點點，整段樓梯就把它抬得很高。疊的臺階越多，爬得越高——這正是「電池越多、電壓越大」的緣由。一節 1.5 伏的 3 號電池，是很短的一段樓梯；電動汽車裡的電池組，則是高高的一段，足有幾百級。

它的位置

伏打的電堆，正是電學其餘部分所需要的那個源頭。它讓奧斯特與安培找到了電流與磁的關聯，讓歐姆（1827）說清了電流、電壓與電阻之間的關係，也讓法拉第（1831）把磁重新變回了電。而伽伐尼那一方——活組織會產生自己的電——也得到了昭雪，這條線一路通向本館中霍奇金—赫胥黎的神經衝動模型（1952）。

The original document

Original source text

A. Volta · Philosophical Transactions of the Royal Society 90 (1800): 403–431 · a letter to Sir Joseph Banks, read 26 June 1800

The letter, written in French and read to the Royal Society on 26 June 1800, announces an apparatus unlike the Leyden jars of the day: instead of a single momentary spark, it furnishes an electric action that is continual and self-renewing. Volta calls it simply his “new instrument.”

The pile

Thirty, forty, sixty, or more pieces of copper, or rather silver, applied each to a piece of tin, or zinc, which is much better, and as many strata of water, or any other liquid which may be a better conductor, such as salt water, ley, &c., or pieces of pasteboard, skin, &c., well soaked in these liquids; such strata interposed between every pair or combination of two different metals in an alternate series, and always in the same order of these three kinds of conductors, are all that is necessary for constituting my new instrument…

From this stack — a disc of silver (or copper), a disc of zinc, a brine-soaked layer, repeated in the same order — Volta draws a current that does not die away. Adding more pairs strengthens the shock; the effect grows with the height of the column.

The crown of cups

Volta also describes a second form, the couronne de tasses (“crown of cups”): a row of cups filled with salt water or dilute acid, each pair joined by an arc of two metals soldered together. It is the same cell unrolled into a chain, and it lays the working part bare — two dissimilar metals separated by a conducting liquid.

[ … ]

Sensations, and the torpedo

The remainder of the letter catalogues what the instrument does to the living body: a persistent taste on the tongue, a flash of light in the eye, shocks felt in the arms when the circuit is closed with wet fingers — effects Volta likens to the natural electric organ of the torpedo ray. The full letter, with these experiments and his argument that the metals themselves are the seat of the electricity, is available at the source below.

Como · March 20, 1800