The Magnetic Effect of a Current

A current always produces a magnetic field around it. This is one of the central facts of physics, and every electromagnet, motor and transformer depends on it.

Around a straight wire the field is a set of concentric circles centred on the wire, in the plane at right angles to it. The circles are closest together near the wire, so the field is strongest there and weakens with distance.

The right-hand grip rule gives the direction: point the thumb of the right hand along the conventional current, and the fingers curl in the direction of the field.

Around a solenoid — a coil of wire — the fields of the individual turns add together, and the result is a field just like that of a bar magnet: emerging from one end, looping round, and entering the other. Inside the solenoid the field is strong and almost uniform.

Which end is the north pole is found with the same right hand: curl the fingers in the direction the current flows round the coil, and the thumb points to the north pole.

Electromagnet — a solenoid with a soft iron core. The core becomes magnetised by the coil's field and greatly strengthens it, and loses that magnetism the moment the current stops.

Three ways to make an electromagnet stronger:

1. Increase the current.
2. Increase the number of turns on the coil.
3. Add or improve the soft iron core.

Bringing the turns closer together also helps.

Where electromagnets are used — scrapyard cranes, electric bells, relays, circuit breakers, loudspeakers and the read-write heads of hard drives. In every case the useful property is that the magnetism can be switched, varied or reversed, which a permanent magnet cannot do.

A relay uses a small current to switch a much larger one. The small current energises an electromagnet, which pulls an iron armature and closes contacts in the high-current circuit. This keeps the dangerous current away from the switch the user touches — which is how a car starter motor is operated from a light ignition switch.

Example: reversing the current through a solenoid swaps its north and south poles, because the field direction reverses with the current. This is exactly what makes an a.c. loudspeaker and a d.c. motor work.