Ørsted's discovery
When an electric current flows through a conductor, a magnetic field is set up around the conductor. That an electric current has a magnetic effect was first shown in 1819 by the Danish scientist Hans Christian Ørsted.
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Activity 13.2 shows that a current in a straight conductor sets up a magnetic effect (a field).
Activity 13.2 — part 1
You will need: a compass, a straight copper wire, a few batteries, connecting wires, a switch, a rheostat
- Put the compass on the table and let its needle settle north–south. Hold the copper wire AB above the compass, along the direction the needle points.
- Connect the batteries and a switch across the ends of AB with connecting wires.
- Close switch S so that a current flows through the wire in the direction AB. The compass needle deflects to the left.
- Stop the current by opening S and watch the needle. It returns to its first position.
- Now put the compass above the wire AB and send the current through AB again. The needle deflects the opposite way.
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Why the compass is the detector
The compass needle is a small magnet, and it moves only when a magnetic field acts on it. With no current it points north, lined up with the Earth's field. When the current flows it swings, so the current must have made a new magnetic field of its own; when the current stops, the needle swings back. Laying the wire along the needle's own direction first matters: the field of a straight wire runs round the wire, so it then acts across the needle and turns it as much as possible. Ørsted's discovery was the first proof that electricity and magnetism are linked — the idea the whole chapter is built on.
A current makes a magnetic field
A compass near a wire swings only while a current flows in the wire (Ørsted, 1819).