Maxwell's corkscrew rule

Two rules can be used to find the direction of the magnetic field around a straight conductor carrying a current. The first is Maxwell's corkscrew rule.

When a corkscrew is turned so that it moves forward in the direction of the current in the conductor, the direction in which the corkscrew turns is the direction of the lines of force of the magnetic field set up by that current.

A corkscrew is a tool used to pull corks out of bottles. An ordinary screw behaves the same way as a corkscrew.

Figure to be added

Figure 13.4 — (a) current flowing up the wire from A to B, with the field circling anticlockwise seen from above; (b) current flowing down from B to A, with the field circling clockwise; (c) a corkscrew driven along a wire: the way it moves gives the current, the way it turns gives the field.

In figure 13.4(a), with the current flowing from A to B, the field is set up anticlockwise.
In figure 13.4(b), with the current flowing from B to A, the field is set up clockwise.

Why a screw is a good model

An ordinary screw goes in when turned clockwise (seen from behind, as you drive it) and comes out when turned anticlockwise. The link between the way it turns and the way it moves never changes, and the link between a current and its field never changes either. So once you tie the forward motion to the current, the turning is always the field. Notice the viewpoint in figure 13.4(a): anticlockwise is as seen from above, looking down on the wire with the current coming towards you. Seen from below, the same field looks clockwise. Always say where you are looking from.

Screw forward with the current, turn with the field

Drive a corkscrew in the direction of the current: the way it turns is the direction of the magnetic field lines.