A wire becomes a magnet
In 1820, a scientist named Oersted watched a compass needle. It moved when he switched a circuit on.
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Step 1 of 3
- Current off. The needle points north.
- Current on. The wire runs north to south. The near needle swings.
- Far from the wire, the needle barely moves.
The rule
A wire with current in it has a magnetic field around it. This is the magnetic effect of electric current. A magnetic field is the space where a magnet's force is felt.
Put a compass near the wire. If the needle turns, there is a magnetic field around the wire. The needle is a small magnet, so the field turns it.
Switch off the current. The field stops, and the needle points north again. To turn the cell round is to swap its two ends. Turn the cell round, and the needle swings the other way.