When you switch on the input circuit (closing switch (1)) the current flows through the solenoid (2).

Inside the solenoid coil is a soft iron core which becomes magnetised only when the current flows.

The solenoid electromagnet attracts the soft iron armature (the pivoted 'rocker') which is rotated anticlockwise.

When the 'rocker' rotates it pushes the contacts at (4) together to close the output circuit.

In this case the output circuit drives an electric motor, but could be anything you want to switch on remotely using a low voltage-current circuit.

 

Uses of a two circuit relay system

(i) This system is used where the output circuit might be operating with a potentially dangerous high p.d. or current.

This is how the starter motor of car is operated. You don't want the high current needed by the starter motor moving through a circuit where you put the ignition key in!

(ii) The output circuit might be in a hazard zone e.g. remote control systems in a nuclear power plant where machinery is operating where there i potentially or actually, radioactive materials - obvious dangers!

 

(c) Electric bell

The d.c. power supply is not shown in the diagram, but the terminal connections are on the left.

When you press the doorbell you close a circuit that allows current to flow to magnetise the soft iron core of the solenoids.

The magnetised soft iron core of the solenoid attracts the striker to hit and ring the bell.

In moving, the striker also breaks the circuit switching off the current and so the electromagnetism of the solenoid.

Therefore the 'sprung' striker then returns to close the circuit, re-magnetising the solenoid soft iron core, so the striker is attracted again to strike the bell.

This happens quite quickly to give a continuous ringing sound.

As long as you press the doorbell, the circuit keeps on being opened and closed to give the bell ringing effect.

(d) Magnetic separators in a recycling plant

 A magnet can be used to pick out scrap iron and steel from a conveyer belt of rubbish.

The items might be cans or steel grills etc. An electromagnet is used for the process.

 

(e) Maglev trains 

(maglev is shorthand for 'magnetic levitation' but not of the spirit world!)

Maglev trains use magnetic repulsion to literally float a train a short height above the guidance track. A magnetic field can be manipulated to move the train along at high speeds with virtually no friction except for air resistance.

Maglev (derived from magnetic levitation) is a system of high train transportation that uses two sets of magnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage of the lack of friction.

With maglev technology, there is just one moving part: the train itself. The train travels along a guideway of magnets which control the train's stability and speed.

Since the propulsion and levitation require no moving parts Maglev trains are quieter and smoother than conventional trains and have the potential for much higher speeds.

Note that electromagnets do not produce a permanent magnetic force. The magnetism they produce is temporary, and can be switched off and on depending upon what is required. Engineers use electromagnetism in the design and construction of maglev trains.
 

(f) Loudspeakers and microphones

Loudspeakers and microphones use an oscillating electromagnet system running off alternating current (a.c.).

For more details see

Motor effect of an electric current including the loudspeaker

Generator effect - applications - including the microphone

 

(g) MRI scanners magnetic resonance imaging

MRI scanners use powerful electromagnets to create detailed images of the inside of your body.

It is a relatively safe technique that does not use ionising radiation, instead it uses safer EM radio waves.

The high frequency radio waves resonate with protons (H atoms) in you body and this resonance is detected and used to build up an image based on where the protons are and their density or concentration - and there are a lot of them in your body e.g. water, fat, protein etc.