物理学 1800

论仅由不同种类导体相互接触所激发的电

亚历山德罗·伏打

把两种金属夹着盐水层层叠起，电第一次源源不断地流。

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In depth · the introduction

两千年来，电一直是一种你造得出、却留不住的火花。伏打找到了让它流动——并且持续流动——的办法。

核心想法

拿两种不同的金属，再拿一块浸了盐水的布。每种金属与湿布相接之处，都会冒出一点点推动电荷的「力」。单独一个这样的「三明治」，那点力小得察觉不到。可一旦把这些三明治朝同一方向叠起来——锌、银、湿布，锌、银、湿布——一点点的推力便会相加。叠上三四十个，你就有了一束稳定的电流，强到足以触到、足以打出火花、足以做功。伏打，就此造出了第一块电池。

成就它的那场争吵

这一切，从一只死青蛙开始。1780 年代，解剖学家路易吉·伽伐尼发现，蛙腿被两种不同的金属碰到时会抽动，于是断定动物自带一种「动物电」。科莫的物理学家伏打先是着迷，继而起疑：他怀疑真正的源头是那一对金属，而非青蛙。

为了证明这一点，伏打干脆把动物整个拿掉。他用湿纸板替下青蛙，把金属圆片层层叠起，直到电堆本身就给出了那记从来不需要青蛙的电击。1800 年 3 月，他把这一切写成一封信，寄给伦敦皇家学会会长约瑟夫·班克斯爵士。电池，正是从两个固执而出色的人长达十年的争论中诞生的——而结果是，两人都对了一半。

它为何重要

能持续的电流，才是你用得上的电流。短短数周之内，别人就用伏打的电堆把水拆成氢和氧，不久又用它撬开「元素」、发现新的元素——化学从此有了电这把手术刀。此后的每一块电池，从手电筒到电动汽车，都是同一个想法：电池排成一列，电压逐个相加。我们衡量这股推力的单位「伏特」，也以他命名。

像一段楼梯

把一个电池想成一级很矮的台阶——单独一级，无足轻重。电堆则是一段楼梯：每一级都把电荷往上抬一点点，整段楼梯就把它抬得很高。叠的台阶越多，爬得越高——这正是「电池越多、电压越大」的缘由。一节 1.5 伏的 5 号电池，是很短的一段楼梯；电动汽车里的电池组，则是高高的一段，足有几百级。

它的位置

伏打的电堆，正是电学其余部分所需要的那个源头。它让奥斯特与安培找到了电流与磁的关联，让欧姆（1827）说清了电流、电压与电阻之间的关系，也让法拉第（1831）把磁重新变回了电。而伽伐尼那一方——活组织会产生自己的电——也得到了昭雪，这条线一路通向本馆中霍奇金—赫胥黎的神经冲动模型（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