The switching noise is usually filtered with an inductor and a capacitor. The impedance may be a resistor, or an inductor (coil). Simulates an inductor (i.e., provides inductance without the use of a possibly costly inductor). Shellac was used for fixing inductor, motor, generator and transformer windings. A variometer is a type of continuously variable air-core RF inductor with two windings. The diode would be in parallel with the series inductor and load. This is called a ferromagnetic core inductor. An inductor consists of a coiled conductor. Polarity of the left side of the inductor is positive. Current from a discharging inductor is one example. The air path ensures that the inductor remains linear. Such an induction loop receiver is classically a very small iron-cored inductor (telecoil). A type of continuously variable air core inductor is the "variometer". In the case of a physical inductor though, the ideal lumped representation is often accurate. This is usually taken to be the constitutive relation (defining equation) of the inductor.