Batteries: from the voltaic pile to the lead-acid cell

Two different metals in a solution that conducts, joined outside by a wire, drive a current: the more reactive metal dissolves and gives up electrons. The pair sets the voltage; the size sets how much current, and for how long.

• Zinc and copper: about 1.1 V in a Daniell cell. A bare zinc–copper cell in brine or acid starts near 1 V and falls fast.
• Iron and copper: about 0.8 V — and iron is easier to get than zinc.
• Zinc and iron: about 0.3 V — not worth building.
• Lead and lead dioxide in sulphuric acid: about 2.0 V, and rechargeable.

Cells in series add voltage — ten Daniell cells give about 11 V. Cells in parallel add current at the same voltage. Bigger plates give more current, never more voltage.

Volta's pile of 1800 is the simplest battery there is:

1. Cut equal discs of zinc and copper, 2–5 cm across, and discs of cloth, card or leather.
2. Soak the separators in saturated brine and squeeze out the excess.
3. Stack zinc, cloth, copper, zinc, cloth, copper… always in the same order. Each zinc–cloth–copper set is one cell of roughly 0.8–1 V.
4. Clamp the stack in a frame and take a wire from each end.

Twenty pairs give roughly 15–20 V — enough to feel on wet fingers and to split water. The pile runs down within hours: hydrogen coats the copper, and the separators dry out. Use it to learn and to test, not to work.

John Daniell's cell of 1836 cures the pile's failing by giving each metal its own solution:

1. A copper vessel, or a copper plate in a glass or glazed jar, filled with saturated copper sulphate solution, with spare crystals on the bottom to keep it saturated.
2. Inside it, an unglazed pot — fired but still porous — filled with dilute zinc sulphate or dilute sulphuric acid, and holding a zinc rod or plate.
3. Take the negative wire from the zinc and the positive from the copper.

It gives 1.1 V, steadily, because copper is deposited on the copper plate instead of hydrogen bubbling there. Cells like this powered the telegraph for decades.

• Copper sulphate comes from dissolving scrap copper in hot dilute sulphuric acid with air bubbled through, or from roasting copper sulphide ore and leaching it with water.
• Keep it working: top up the crystals, replace the zinc as it wears, and replace the pot once copper starts to deposit on the zinc.

Zinc is the hard metal here. Its ores — calamine (zinc carbonate) and sphalerite (zinc sulphide, roasted first) — reduce with charcoal only above about 1,000 °C, but zinc boils at 907 °C. In an open furnace it leaves as vapour and burns to white oxide, so it must be distilled:

1. Roast sulphide ore in air to turn it to oxide — outdoors, because the sulphur leaves as choking sulphur dioxide.
2. Mix the oxide with charcoal and pack it into sealed clay retorts that lead down into a cooler receiver.
3. Heat the retorts to about 1,100–1,200 °C. Zinc vapour condenses in the cooler receiver as liquid metal.

Zawar in Rajasthan distilled zinc this way on an industrial scale by the 14th century. In Europe, William Champion patented the process in 1738 and built the first zinc works near Bristol in the 1740s.

If zinc cannot be made, use iron: an iron–copper cell gives about 0.8 V from metal already in hand.

A current in a wire swings a compass needle — Ørsted's discovery of 1820, and the first electrical measuring instrument:

1. Wind 20–50 turns of insulated copper wire — cotton-wrapped, silk-wrapped or varnished — round a small frame.
2. Set a compass inside or under the coil, and turn the frame so the wire runs north–south, along the needle.
3. A current through the coil swings the needle: more current, more swing.

Compare cells by how far they swing it; a cell that swings it less than its twin is failing. Use a few turns of thick wire for large currents and many turns of thin wire for small ones.

Gaston Planté's cell of 1859 can be recharged, which none of the cells above can:

1. Two lead sheets, kept apart by strips of cloth or rubber, stacked or rolled, in dilute sulphuric acid of specific gravity about 1.25 — roughly 1 part concentrated acid to 3.5 parts water by volume, the acid poured into the water.
2. Form the plates: charge from another source — Daniell cells, or later a generator — discharge, and charge again, many times. One plate turns to brown lead dioxide (positive), the other to grey spongy lead.
3. Charge at about 2.4 V per cell, and stop when both plates bubble freely. A charged cell rests at about 2.1 V.

Read the charge with a hydrometer: the acid grows denser as the cell charges — about 1.28 full, 1.12 flat.

• Copper plating: hang the object to be coated on the negative wire and a copper plate on the positive, both in copper sulphate solution with a little sulphuric acid. Clean the object first — degreased, scrubbed, dipped in dilute acid — or the coat peels. One Daniell cell is enough; too much current gives a dark, powdery coat instead of bright pink copper.
• Splitting water: two electrodes in water made conductive with washing soda (sodium carbonate) or a little sulphuric acid. Hydrogen collects at the negative, oxygen at the positive, twice as much hydrogen by volume. Never let the two gases mix, and never use salt — it gives off poisonous chlorine at the positive electrode.
• The telegraph: a battery, a key, a line and a sounder — see telegraph.