Chemistry 1834

Experimental Researches in Electricity, Seventh Series

Michael Faraday

Chemical change is measured out by charge.

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In depth · the introduction

Pass electricity through a chemical and it comes apart — and Faraday found that exactly how much comes apart is set, to the milligram, by how much electricity you send.

The big idea

Send a current through a liquid that holds a dissolved compound and the compound splits, its pieces collecting on the two metal plates dipped in it — this is how a cheap fork gets a real coat of silver. Faraday's discovery was that the process is not vague but exact: double the electricity and you get exactly double the metal. The amount deposited is proportional to the total charge that flows.

He found a second rule too. The same dose of electricity sets free different elements in proportion to their “equivalent weight.” Silver, heavy for each unit of charge it needs, plates out in far greater mass than light aluminium for the very same current. Charge had become a thing the chemist could weigh.

How it came about

In the early 1830s Faraday — once a bookbinder's apprentice, by then the star experimenter at London's Royal Institution — was untangling whether the electricity from a battery, a spark machine, a magnet and an electric fish were the same thing. He built small cells that recorded how much electricity had passed through them by weighing the metal, or measuring the gas, they produced.

Cell after cell, in solution after solution, one rule held: chemical change tracked the charge and nothing else. To describe what he was seeing he needed words the old theories did not own, so he wrote to the Cambridge scholar William Whewell, who coined them — ion, anode, cathode, electrode, electrolyte — terms now spoken in every chemistry class.

Why it mattered

It made electricity measurable on the chemist's balance, and turned electroplating, metal refining and battery-making into exact crafts rather than guesswork. And the strange precision — one fixed dose of electricity for each “equivalent” of matter — was an early hint that electricity itself comes in countable units. Decades later that hint became the electron.

A way to picture it

Think of it as a vending machine for atoms. Each coin is one unit of charge, carried by one electron. A silver ion costs one coin to release, a copper ion two, an aluminium ion three. Feed in a fixed number of coins and you can say in advance precisely how many atoms of each metal will drop out — and because atoms of different elements weigh different amounts, the same coins buy very different masses. Faraday's law is the machine's exact price list.

Where it sits

Volta's battery of 1800 had made a steady current available, and Humphry Davy had used it to tear compounds apart and isolate sodium and potassium. Faraday, Davy's former assistant, turned that qualitative power into a law. His fixed charge-per-equivalent points forward to Stoney and Thomson's electron and to Nernst (1889), who would put a number on the voltage these same ions produce — and the vocabulary Faraday coined still labels the anode, cathode and electrolyte of every battery you own.

The original document

Original source text

Michael Faraday · Philosophical Transactions of the Royal Society of London 124 (1834): 77–122 · Royal Institution, London

The Seventh Series is the paper in which electrolysis became quantitative. Faraday had spent the previous year showing that the electricity of a voltaic battery, a friction machine, a magnet and an electric fish are one and the same agent; here he asks how much chemical work a given amount of it performs — and answers: a definite, weighable amount.

1 · A meter made of chemistry

First he needed to measure electricity itself. His “volta-electrometer” is a cell in which the gas evolved, or the metal deposited, records the total quantity of electricity that has passed. Placed in series in a circuit, identical cells always showed identical amounts — the measure did not depend on the size of the plates, the strength of the solution, or the kind of generator.

2 · Definite electro-chemical action

From cell after cell he drew a single law of definite action: the chemical effect is fixed by the quantity of electricity, and by nothing else.

…the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes.

And when the same current was sent through different electrolytes, the weights set free stood in the ratio of what Faraday called their electro-chemical equivalents — numbers that proved to coincide with the ordinary chemical equivalents already known to chemists. In modern symbols the whole result is m = (Q·M)/(z·F), with the Faraday constant F = 96485 coulombs per mole.

3 · A new language for electrolysis

To describe a process he pictured as matter travelling through the liquid to the plates — not as an attraction reaching out from the “poles” — Faraday wanted words free of the old theory. With the Cambridge polymath William Whewell he coined them: the plates became the electrode, the anode and the cathode; the decomposed substance an electrolyte; and the migrating particles ions, dividing into anions and cations.

I propose to distinguish these bodies by calling those anions which go to the anode of the decomposing body; and those passing to the cathode, cations; and when I have occasion to speak of these together, I shall call them ions.

[ … ]

Faraday measured one fixed dose of electricity for each “equivalent” of matter, but did not claim what carried it. Half a century later Helmholtz read in these very numbers the existence of “atoms of electricity,” and the electron — the coin Faraday had unknowingly been counting — was named by Stoney and found by J. J. Thomson.

Royal Institution, London · 1834