I wondered whether one could use the static electricity of the Wimshurst machine to light up one of Uncle Dave’s lightbulbs. Uncle said nothing, but provided me with some very fine wire made of silver and gold only a three-hundredth of an inch thick. When I connected the brass balls of the Wimshurst machine with a three-inch length of silver wire on a card, the wire exploded when I turned the handle, leaving a strange pattern on the card. And when I tried it with the gold wire, this was vaporized instantly, turning into a red vapor, gaseous gold. It seemed to me from these experiments that frictional electricity could be quite formidable – but that it was too violent, too intractable, to be of much use.
Electrochemical attraction, for Davy, was the attraction of opposites – the attraction, for example, of an intensely ‘positive’ metallic ion, a cation like that of sodium, to an intensely ‘negative’ one, an anion like that of chloride. But most elements, he thought, came between these on a continuous scale of electro-positivity or -negativity. The degree of electro-positivity among metals went with their chemical reactivity, hence their ability to reduce or replace less positive elements.
This sort of replacement, without any clear notion of its rationale, had been explored by the alchemists in the production of metallic coatings or ‘trees.’ Such trees were made by inserting a stick of zinc, say, into a solution of another metallic salt (a silver salt, for example). This would result in the displacement of the silver by the zinc, and metallic silver would be precipitated from the solution as a shining, almost fractal, arborescent growth. (The alchemists had given these trees mythical names, so the silver tree was called Arbor Dianae, the lead tree Arbor Saturni, and the tin tree Arbor Jovis.)[34]
I had hoped, at one point, to make such trees of all the metallic elements – trees of iron and cobalt, and bismuth and nickel, of gold, of platinium, of all the platinium metals; of chromium and molybdenum, and (of course!) tungsten; but various considerations (not least, the prohibitive cost of the precious metal salts) confined me to a dozen or so basic ones. But the sheer aesthetic delight of these – no two trees ever looked the same; they were as different, even with the same metal, as snowflakes or ice crystals, and different metals, one could see, were deposited in different ways – soon gave way to a more systematic study. When did one metal lead to the deposition of another? And why? I used a zinc rod, sticking it first into a solution of copper sulfate, and got a gorgeous encrustation, a copper plating, all around it. I then experimented with tin salts, lead salts, and silver salts, putting a zinc rod into solutions of these, and produced shining, crystalline trees of tin, lead, and silver. But when I tried to make a zinc tree, by sticking a copper rod into a solution of zinc sulphate, nothing happened. Zinc was clearly the more active metal, and as such could replace the copper, but not be replaced by it. To make a zinc tree, one had to use a metal even more active than zinc – a magnesium rod, I found, worked well. Clearly all these metals did form a sort of series.
Davy himself pioneered the use of electrochemical displacement for protecting the copper bottoms of ships from corrosion in seawater, attaching to them plates of more electropositive metals (such as iron or zinc), so that these would become corroded instead, a so-called cathodic protection. (Though this seemed to work well under laboratory conditions, it did not work well at sea, because the new metal plates attracted barnacles – and thus Davy’s suggestion was ridiculed. Yet the principle of cathodic protection was brilliant, and eventually became, after his death, a standard way of protecting the bottoms of ocean-going vessels.)
Reading about Davy and his experiments stimulated me to a variety of other electrochemical experiments: I put an iron nail in water, attaching a piece of zinc to it to protect it from corrosion. I removed the tarnish from my mother’s silver spoons by putting them in an aluminium dish with a warm solution of sodium bicarbonate. She was so pleased by this that I decided to go further and try electroplating, using chromium as the anode and a variety of household objects as the cathode. I chromium-plated everything I could lay hands on – iron nails, bits of copper, scissors, and (this time to my mother’s considerable annoyance) one of the silver spoons that I had previously cleaned of tarnish.
I did not realize at first that there was any connection between these experiments and the batteries I was playing with at the same time, although I thought it an odd coincidence that the first pair of metals I used, zinc and copper, could produce either a tree or, in a battery, an electric current. I think it was only when I read that, to get a higher voltage, batteries used nobler metals such as silver and platinum that I started to realize that the two series – the ‘tree’ series and Volta’s series – were probably the same, that chemical activity and electrical potential were in some sense the same phenomenon.
We had a large old-fashioned battery, a wet cell, in the kitchen, hooked up to an electric bell. The bell was too complicated to understand at first, and the battery, to my mind, was more immediately attractive, for it contained an earthenware tube with a massive, gleaming copper cylinder in the middle, immersed in a bluish liquid; all this inside an outer glass casing, also filled with fluid, and containing a slimmer bar of zinc. It looked like a miniature chemical factory of sorts, and I thought I saw little bubbles of gas, at times, coming off the zinc. This Daniell cell (as it was called) had a thoroughly nineteenth-century, Victorian look about it, and this extraordinary object was making electricity all by itself – not by rubbing or friction, but just by virtue of its own chemical reactions. That this was quite another source of electricity, not frictional or static, but a radically different sort of electricity, must have seemed astounding in the extreme, a new force of nature, when Volta discovered it in 1800. Previously there had been only the fugitive discharges, the sparks and flashes, of frictional electricity; now one could have at one’s disposal a steady, uniform, unvarying current. One only needed two different metals – copper and zinc would do, or copper and silver (Volta worked out a whole series of metals, differing in the ‘voltage’, the potential difference, between them), immersed in a conducting medium.
The first batteries I made myself used fruit or vegetables – one could stick copper and zinc electrodes into a potato or a lemon and get enough current to light a tiny 1-volt bulb. And one could wire half a dozen lemons or potatoes together (in series to get a higher voltage, or in parallel to get more power) to make a biological ‘battery.’ After the fruit and vegetable batteries, I turned to coins, using alternating copper and silver coins (one had to use silver coins made before 1920, for later ones were debased) with moistened (usually saliva-moistened) blotting paper between them. If I used small coins, farthings and sixpences, I could get five or six such couples in an inch, or I could make a pile a foot high, with sixty or seventy couples, enclosed in a tube, which could give quite a sharp, 100-volt shock. One could go on, I thought, to make an electric stick filled with narrow couples of copper and zinc foil, a lot thinner than coins. Such a stick, with five hundred or more couples, might generate a thousand volts, more even than an electric eel, enough to frighten off any assailant – but I never got as far as making one.
I was fascinated by the huge range of batteries developed in the nineteenth century, some of which I could see in the Science Museum. There were ‘single fluid’ batteries, like Volta’s original cell, or the Smee, or the Grenet, or the massive Leclanche, or the slim, silver battery of de la Rue; and there were two-fluid batteries, like our own Daniell, and the Bunsen, and the Grove (which used platinum electrodes). Their number seemed endless, but all were designed, in their different ways, to secure a more reliable and constant flow of current, to protect the electrodes from the deposition of metal or the adherence of gas bubbles, and to avoid (as some batteries caused) the emission of noxious or inflammable gases.
34
These names for metallic trees came from the alchemical notion of the correspondence between the sun, the moon, and the five (known) planets with the seven metals of antiquity. Thus gold stood for the sun, silver for the moon (and the moon goddess, Diana), mercury for Mercury, copper for Venus, iron for Mars, tin for Jupiter Qove), and lead for Saturn.