Inductive coil is wound on the insulating tube by the wire circle by circle. The wires are insulated from each other. The insulating tube can be hollow, or it can contain iron core or magnetic particle core. The inductance of the coil is expressed in L, in Henry (H), milli Henry (mH) and milli Henry（ μ H)，1H=10^3mH=10^6 μ H。 It uses the principle of electromagnetic induction to work. When a current flows through a wire, a certain electromagnetic field will be generated around the wire, and the wire of the electromagnetic field itself will induce the wire within the electromagnetic field range. The effect on the wire itself that generates the electromagnetic field is called "self induction", that is, the changing current generated by the wire itself generates the changing magnetic field, which further affects the current in the wire; the effect on other wires within the electromagnetic field range is called "mutual inductance". At the same time, inductance coil is also one of the commonly used components in electrical products, instrument equipment and other computers. It is an electronic component that uses the working principle of magnetic effect of current to carry out operations. Its electrical characteristics and power capacitor conversely, "pass low frequency, resistance high frequency". When the high-frequency data signal is based on the inductance coil, it will encounter very large friction resistance, so it is not easy to rely on it; For low-frequency information, the friction resistance is relatively small, that is, high-frequency electromagnetic wave can be easily based on it. The resistance of inductance coil to DC power supply is basically zero Classified according to the form of inductance: fixed inductance, variable inductance. Classification according to the properties of magnetic conductors: air core coil, ferrite coil, iron core coil, copper core coil. Classification according to the nature of work: antenna coil, oscillating coil, choke coil, notch coil, deflection coil. Classification according to winding structure: single-layer coil, multi-layer coil, honeycomb coil, closely wound coil, intermittent wound coil, bodiless coil, honeycomb coil, random wound coil