Ampère's right-hand grip rule

Ampère's right-hand grip rule is another, easier rule for finding the direction of the magnetic field set up when a current flows through a conductor.

If the conductor is gripped in the right hand with the thumb pointing in the direction of the current, the curled fingers show the direction of the magnetic field around the conductor.

Figure to be added

Figure 13.5 — a right hand gripping a wire: the thumb points along the current, the four curled fingers show the direction of the field.

Drawing a field into or out of the page

Figure 13.6 shows how the field of a current in a wire is drawn. Suppose the current flows in the direction AB. By the right-hand rule, above the wire the field comes out of the paper towards you, and below the wire it goes into the paper. A field coming out of the paper is drawn as a dot in a circle (⊙); a field going into the paper is drawn as a cross in a circle (⊗).

Figure to be added

Figure 13.6 — a wire with current flowing from A to B across the page: ⊙ marks above the wire (field out of the page), ⊗ marks below it (field into the page).

Why the dot and the cross

Think of an arrow. Coming straight at you, you see its sharp point — a dot. Flying away from you, you see its tail feathers — a cross. The same marks are used for a current as well as a field: a wire seen end-on with ⊙ carries current out of the page, and with ⊗ into it. Gripping such a wire with the right hand, thumb out of the page, the fingers curl anticlockwise; thumb into the page, clockwise. You will need these marks for Fleming's rules on the next pages, where the current and the field are always drawn at right angles.

Right hand, not left

Using the left hand gives the field exactly the wrong way round. The grip rule always uses the right hand.

Thumb along the current, fingers round with the field

Grip the wire in the right hand, thumb along the current: the fingers curl the way the field goes. ⊙ means out of the page, ⊗ into it.