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An inductor is a coil of wire. Inductors have both ac and dc resistance. ac resistance is known as impedance. The impedance of an inductor is much greater than the inductor's dc resistance.
When electrons flow through a wire, they produce an electro-magnetic field, abbreviated emf. In a coil, the turns of wire will produce an emf field that will slightly impede, or resist electron flow of a dc current. When an ac current is used, the ac impedance is very high, compared to the dc resistance. This is because the ac signal is constantly creating an electro magnetic field that is produced has a polarity on it. An ac signal starts at zero volts then begins to climb to its full voltage. At the point that the ac signal reaches its peak voltage, a magnetic field has been built up in the inductor. As the ac signal voltage begins to fall to zero volts, this built up magnetic field begins to pull electrons through the wire, which is known as the generator effect. When the ac signal crosses the zero volt line and begins to conduct in the opposite direction, the magnetic field is trying to pull the electrons the opposite direction. This creates a lot of ac impedance.
A common electronics term is, the current leads and the voltage lags n an inductor, abbreviated, ELI. This means that when viewing the sine waves for the voltage and the current of an ac signal through an inductor, the voltage level will be out of phase with the current, on the time divisions, it will be a few steps ahead of the current sine wave. The current will follow what the voltage does, but will lag behind it, because it takes work to create a magnetic field.
Inductors are used as filters, transformers, and resonators. In a transformer, two coils are wound onto the same metal conductor, and the voltage and current from one coil is transferred to the other coil. By varying the number of turns on the secondary coil, voltage can be stepped up or down. Inductors are often used with capacitors because capacitors have an opposite effect; current leads and voltage lags in a capacitor. In a tuner circuit and in class C amplifiers, a capacitor will be connected in parallel with an inductor, and will produce a tank circuit that will resonate at a specific frequency.
Basically, there are air core inductors, and inductors with a conductor for a core. The conductor can be iron, ferrite, or some other type of metal.
Hamfile's note is short & sweet. Very informative. Let me add some things for the benefit of readers.
The unit of inductance is the 'henry (plural,henrys;not henries),named after Joseph Henry, the American physicist. A circuit has an inductance of 1 henry if a current which is changing at the rate of 1 ampere per second produces in it a back emf of 1 volt.
The inductance of a coil is dependent upon the linkage between truns and flux. In a single-turn coil the linkage is equal to the total flux,since all the flux links with the single turn. In a coil with two turns there is twice as much flux and also twice as many turns for it to link with,so that the inductance is four times as great. Provided that every line of force links with every turn,we can say that in general the inductance is proportional to the square of the number of turns. Actually, this condition is not always attained in practice,particulary in the case of long coil such as a soleniod,and some of the flux links with less than the full number or turns.
The henry is an inconveniently large unit for many purposes,and a subdivision,the microhenry,is commonly used.one henry equals one million microhenrys.
Inductances,like resistances,can be connected in series or in parallel.Provided that they have no mutual inductance,their joint value may be found by the methods used for resistances.
The color coded ones are decoded the same way resistors are. The values are in microhenrys. There are more tolerances for the 4th band than for resistors, and sometimes there are 5 bands. If 5 bands, the first band will be a fatter, wider band. Start with this one on the left, skip it and read from left to right. Here's a link to a good color code chart.