The inductor

Inductance coil (inductor) is a wire wound around a metal core. When the current passes through the wire, the core is magnetized. Magnetic field in the coil interacts with the current, which flows through it, prevents any change of the current. As a result, the magnitude of direct current flowing through the inductor is limited to a relatively low resistance wire. But AC or pulsed DC coil encounters stronger resistance. This effect is used in some power supplies constant voltage. The current which passes through the coil, eliminates voltage fluctuations, that is, provide "aligned" power. A typical transformer contains two windings wound on one core. AC voltage applied to the first (primary) winding creates an alternating magnetic field which induces an alternating voltage in the secondary winding. The voltage on it can be more or less from a primary dependence on the ratio of the number of turns on the coils. Thus, the device transforms one AC voltage to another.
Resonant circuit consists of a capacitor in the circuit parallel to the inductor. This chain is used for selection of signals or tuning in radio and television circuits. At a certain frequency, called resonance, report signal generates a relatively large voltage in this circuit. To select another station, you must change the resonant frequency of the resonant circuit i.e. change the capacitance, or inductance, or connected to another resonant circuit. The capacitors are manufactured in the form of a set of metal plates. Revolving, they are shuffled relative to each other, which changes the area of overlap of the plates. Coil variable inductance cores have a metal that you can screw or Unscrew.