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When there is a current in a wire, a circular magnetic field is produced around the wire. This magnetic field can deflect the needle of a magnetic compass. The strength of the magnetic field is greater:

• closer to the wire
• if the current is increased

Solenoids

A solenoid is a long coil of wire. When a direct electric current is passed through it, the shape of the magnetic field is very similar to the field of a bar magnet. The field inside a solenoid is strong and uniform. The small magnetic fields caused by the current in each turn of the coil add together to make a stronger overall magnetic field.

The strength of the magnetic field around a solenoid can be increased by:

• increasing the number of turns on the coil
• increasing the current
• placing an iron core inside the solenoid

The use of an iron core makes the solenoid into an electromagnet which can be switched on and off.