Electromagnetic coils, most commonly known as inductors, are amongst the simplest electric and electronic components. They basically consist of a simple metal electrically insulated wire looped into a cylindrical, toroidal or even disk-like shape, with the role of providing inductance in an electric circuit. Inductance is an electrical property characteristic to electromagnetic coils opposing the flow of current through the circuit.
How it works
In order to understand the workings of inductors, one must first imagine a simple electrical circuit consisting of an incandescent light bulb connected in parallel to an inductor. This circuit is powered through a battery also connected in parallel to the light bulb through a switch. Let's say we hit the switch. Common sense tells us that because the coil has a lower electrical resistance than the light bulb, it will act as a short circuit and prevent the light bulb from emitting light.
However, the reality is very different. When the switch is turned on, the light bulb burns brightly at first before dimming to a lower light intensity. The same effect takes place when the switch is turned off, only that after experiencing high brightness the light bulb stops emitting light completely.
This is due to inductance. When current starts flowing through the coil it generates a magnetic field that tries to stop the current from flowing through the coil by generating a second current but in opposite direction. Albeit, when the magnetic field is established, the current flow goes back to normal. Alternatively, when the current flow is stopped, the magnetic field tries to compensate in order to maintain it by generating electric current through the coil. By doing so the magnetic field can no longer be sustained and collapses, keeping the light bulb lit for just a small amount of time.
The inductance of an electromagnetic coil is measured in Henries - after the American scientist Joseph Henry who discovered electromagnetic induction roughly at the same time as Michael Faraday. The inductance of a coil is calculated using an equation involving the number of turns, the properties of the coil core - air, vacuum, iron, etc. - the cross section of the turns and coil length.
Electromagnetic coils are generally used either to generate magnetic fields or as inductors inside electronic oscillators and can be found mostly anywhere around an electric or electronic device.