Galvani’s frog

Two metals, and no frog

At Pavia, Alessandro Volta is won over, then notices by 1792 what Galvani had seen without keeping it: an arc of two different metals contracts strongly, one of a single metal little or not at all. The frog is for him only an electrometer; the source is the contact of the metals. He shows it without an animal, by an old trial of Sulzer’s: a strip of tin on the tongue, a silver coin farther back, and as soon as they touch, a sour taste, as long as the contact lasts. Volta ranks the metals and calls electromotive force what drives the fluid at the contact.

Galvani replies in 1794 with a contraction without any metal, the nerve of one leg folded back onto its own muscle, where it is cut. The fact is true; Volta explains it badly. Giovanni Aldini, Galvani’s nephew, takes up the quarrel. Volta stacks a disc of zinc, a disc of silver or copper, a round of cardboard soaked in brine, twenty times, forty times; the column shocks at will, without rubbing or rest. He also lines up cups of salt water joined by arcs of zinc and silver, his crown of cups, and the shocks add up. On March 20, 1800, he describes both in a letter to Joseph Banks, president of the Royal Society, read on June 26 and printed in the Philosophical Transactions.

What became of them

Galvani never saw the pile. He refuses the oath to the Cisalpine Republic, loses his chair and dies in December 1798. Aldini takes galvanism around Europe; in London, in January 1803, he passes the current of a pile through the corpse of George Forster, and the hanged man’s jaw moves. Volta shows his pile at the Institut in Paris in 1801, before Bonaparte; he will be a count, then, in 1881, the volt. Banks circulates the letter before it is read; William Nicholson and Anthony Carlisle build a pile from it. In early May 1800, its two wires dipped in water give hydrogen and oxygen: the water is decomposed. Faraday, in 1834, gives this chemistry its words, with Whewell’s help: electrode, electrolyte, ion.

The cell, as we read it today

Volta was right about the two metals and wrong about the cause. Contact alone gives no lasting current; some material has to be consumed, and in the pile of 1800 it is the zinc. Each atom that goes into the brine as an ion leaves two electrons on the disc; they go off through the wire to the silver, where the hydrogen of the water takes them and makes bubbles. The bubbles insulate the silver and the pile weakens: that is polarization. In the wet cardboard, the ions close the loop the electrons began in the wire; no electron crosses the electrolyte, no ion passes into the metal. That is the complete loop lesson 1.1 has you draw: ions inside, electrons outside.

The voltage comes from there. Each metal in its solution has its electrode potential; the cell gives the difference: about a volt for zinc and copper, 3.7 V between lithium-filled graphite and the oxide facing it in a tool pack. Kuphaldt puts the cell this way: two electrodes, an electrolyte, a potential proper to the pair. Lithium-ion is only another pair, in an electrolyte without water, because water decomposes at little more than a volt; Nicholson and Carlisle saw it come apart first. Putting cells in series is rebuilding the crown of cups.

Galvani was not wrong about everything. The nerve is excitable, and the contraction without metal was a real current, the one that leaks from a cut muscle. Nobili reads it on a galvanometer in 1828 without recognizing it, Matteucci measures it around 1840, du Bois-Reymond follows it from 1843 into the nerve, and a century later the action potential is written down. The living cell keeps across its membrane, by a pump, an imbalance of sodium and potassium, a few tens of millivolts: it is itself a cell run on ions. Galvani had the right idea in the wrong place.

The words that remain

The volt, adopted in 1881 at the Paris congress, measures what Volta tasted on his tongue. Galvani left his name to galvanism, Volta’s word, to the verb galvanize, first to shock a body with the pile, then to rouse a crowd, finally to zinc-coat iron, and to the galvanometer. French kept Volta’s stack, la pile, and says it of a single cell. English kept battery, the word Franklin gave his row of lead-covered panes, then of Leyden jars, which slid onto Volta’s pile. In English the cell is a cell, the stack a pile, the assembly a battery; at the bench, in either language, it is the pack.