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Coil usually refers to the wire winding in the shape of a ring, the most common coil applications are: motor, inductor, transformer and ring antenna, etc. The coil in a circuit is the inductor. The wires are wound one by one and insulated from each other. The insulated tube may be hollow or contain an iron core or magnetic powder core, called inductance for short. Inductance can be divided into fixed inductance and variable inductance, fixed inductance coil referred to as inductance or coil. Denoted by L, the units are Henry (H), millimeter Henry (mH), micro Henry (uH),1H=10^3mH=10^6uH.