A plane coil is a single turn of wire made into a circle. When an electric current flows, the magnetic field forms into circles around opposite sides of the circle. The field is strongest inside the coil.
A solenoid consists of a wire coiled up into a spiral shape. When an electric current flows, 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 coil add together to make a stronger overall magnetic field.
A solenoid with an iron core is called an electromagnet. The iron core increases the solenoid's magnetic field strength. A simple electromagnet is made by coiling wire around an iron nail.
Electromagnets are used in devices such as electric bells, and door locks that can be controlled remotely.
Magnetic field strength is often called magnetic flux density and is given the symbol B.
Magnetic field strength is defined as the force acting per unit current in a wire of unit length, which is perpendicular to the field, and it is measured in tesla (T).
A magnetic field has a strength of one tesla. This is if a wire with a current of one amp and is one metre long experiences a force of one newton.