Magnetic effect of current

A compass needle is itself a tiny magnet. It turns until it lies along the magnetic field at its position, so its behaviour can reveal a field that cannot be seen directly.

The field around a straight wire

Around a long, straight current-carrying wire, the magnetic field forms concentric circular loops centred on the wire. In a plane perpendicular to the wire, every field line is therefore a circle rather than a line pointing towards or away from the conductor.

The direction of the field at any point is along the tangent to its circular field line. This is why plotting compasses placed at different positions around the wire point in different directions: each needle lies along a different part of the circular pattern.

Finding the field direction

Use the right-hand grip rule. Grip the wire with your right hand so that your thumb points in the direction of conventional current. The way your fingers curl gives the direction of the magnetic-field loops.

When a wire is drawn end-on, a dot, $\odot$, means that the current comes out of the page. The right-hand rule then gives an anticlockwise magnetic field. A cross, $\otimes$, means that the current goes into the page, giving a clockwise field. Reversing the current therefore reverses every field arrow, although the field lines remain circular.

Demonstrating the magnetic effect

A suitable investigation uses a straight wire passing vertically through a horizontal card, a low-voltage d.c. supply, a switch and one or more plotting compasses.

1. With the switch open, place a plotting compass near the wire and note the needle direction. This provides a comparison with the background magnetic field.
2. Close the switch so that current flows. The compass needle deflects, showing that the current has produced a magnetic effect around the wire.
3. Put the plotting compass at several marked positions around the wire. At each position, mark the direction in which its north-seeking end points.
4. Join the direction marks smoothly. They trace circular field lines centred on the wire; add arrowheads to record the direction.
5. Reverse the connections to the supply. The conventional current reverses and the compass deflections reverse, confirming that the field direction depends on the current direction.

Opening the switch again should remove the field due to the wire, so the compass returns towards its original direction. This comparison helps show that the deflection was caused by the current rather than by the unconnected wire or the card.

What the evidence shows

One deflected compass demonstrates that a current can create a magnetic effect. Measurements at several positions reveal the field's shape and direction. Together, the observations support three conclusions: the field surrounds the conductor, its lines are concentric circles, and its direction reverses when the current reverses.