The Voltaic cell

Cells have lit our lamps, made our magnets and heated our wires. But what inside a cell makes electricity? Start with one of the very first.

A Voltaic cell (also called a Galvanic cell) has:

• two metal rods of different metals, called electrodes;
• a liquid called an electrolyte, usually a weak acid or a salt solution;
• a glass or plastic container holding it all, with the rods partly dipped in the liquid.

A chemical reaction between the rods and the electrolyte makes electricity: chemical energy becomes electrical energy. Connect a circuit and current flows from the positive terminal, round the circuit, to the negative terminal.

In time the chemicals are used up, like a bottle running dry, and the cell stops working. It is then called dead.

Make one in the kitchen (Activity 4.6). Push a copper wire and an iron nail into each of five or six juicy lemons, a little apart. Join the copper wire of one lemon to the nail of the next. Connect an LED between the copper of the first lemon and the nail of the last. Here the copper and iron are the electrodes, and the lemon juice is the electrolyte (salt water works too).

If the LED doesn't light, turn it round: from Grade 7, current passes through an LED only when its longer leg goes to the positive side.

Where the names come from. In the late 1700s the Italian scientist Luigi Galvani saw a dead frog's leg kick when touched with two different metals, copper and iron. Everyone knew electricity could make muscles move, and Galvani decided the electricity came from the frog.

Alessandro Volta, also Italian, disagreed: he thought it came from the metals. To test it, he left the frog out completely and used paper soaked in salt water instead. He still got a current. So it was the two metals and the liquid that made electricity, and that led to the first battery.