The terms coil and inductance are often used synonymously, with the difference that the coil is a wound wire which can be surrounded by iron cores or other magnetically conductive materials such as ferrite for better conduction of the magnetic field lines.
The number of turns and the magnetic conductivity, or permeability, of the ferromagnetic material are critical to the inductance of coils. Other factors that influence inductance are the winding and conductor diameters and the distance between the windings. Important characteristics of coils are inductance tolerance, temperature stability, dielectric strength and quality factor (Q).
Coils are available in a wide variety of techniques, designs and shapes with fixed values or as a variable coil, without and with a ferromagnetic core or with a completely enclosing ferromagnetic housing. Among the coil techniques, a distinction is mainly made between the wound coil, wirewound inductance, and multilayer inductances. In addition, there are thin-film, thick-film and metal-composite coils, which are also designed as multilayer inductors for SMT technology. As for the designs, there are the air core coil, inductor, pot coil and flat coil. They can be found in relays and in combination with resistors in filters, furthermore together with capacitors in filter circuits and in oscillating circuits.