page_title: Inductor
section_title: Variable inductor
hierarchical_section_title: Inductor / Types / Variable inductor
caption_attribution_description: English: An electronic component called a roller inductor, a type of air-core RF inductor that can be adjusted continuously. It consists of a coil of wire that can be rotated with an adjustment knob, with an electrical contact in the form of a grooved wheel that rides on the wire turns. Rotating the inductor causes the contact wheel to move up or down the length of the coil, allowing more or fewer turns of the coil into the circuit, thus increasing or decreasing the inductance. Roller inductors are used mainly in tuned circuits in electronic oscillators, such as radio transmitters to tune the transmitter to different frequencies.
context_page_description: An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coil around a core.
  When the current flowing through an inductor changes, the time-varying magnetic field induces an electromotive force in the conductor, described by Faraday's law of induction. According to Lenz's law, the induced voltage has a polarity which opposes the change in current that created it. As a result, inductors oppose any changes in current through them.
  An inductor is characterized by its inductance, which is the ratio of the voltage to the rate of change of current. In the International System of Units, the unit of inductance is the henry named for 19th century American scientist Joseph Henry. In the measurement of magnetic circuits, it is equivalent to weber/ampere. Inductors have values that typically range from 1 µH to 20 H. Many inductors have a magnetic core made of iron or ferrite inside the coil, which serves to increase the magnetic field and thus the inductance.
context_section_description: Probably the most common type of variable inductor today is one with a moveable ferrite magnetic core, which can be slid or screwed in or out of the coil. Moving the core farther into the coil increases the permeability, increasing the magnetic field and the inductance. Many inductors used in radio applications (usually less than 100 MHz) use adjustable cores in order to tune such inductors to their desired value, since manufacturing processes have certain tolerances (inaccuracy). Sometimes such cores for frequencies above 100 MHz are made from highly conductive non-magnetic material such as aluminum. They decrease the inductance because the magnetic field must bypass them.
  Air core inductors can use sliding contacts or multiple taps to increase or decrease the number of turns included in the circuit, to change the inductance. A type much used in the past but mostly obsolete today has a spring contact that can slide along the bare surface of the windings. The disadvantage of this type is that the contact usually short-circuits one or more turns. These turns act like a single-turn short-circuited transformer secondary winding; the large currents induced in them cause power losses.
  A type of continuously variable air core inductor is the variometer. This consists of two coils with the same number of turns connected in series, one inside the other. The inner coil is mounted on a shaft so its axis can be turned with respect to the outer coil. When the two coils' axes are collinear, with the magnetic fields pointing in the same direction, the fields add and the inductance is maximum. When the inner coil is turned so its axis is at an angle with the outer, the mutual inductance between them is smaller so the total inductance is less. When the inner coil is turned 180° so the coils are collinear with their magnetic fields opposing, the two fields cancel each other and the inductance is very small. This type has the advantage that it is continuously variable over a wide range. It is used in antenna tuners and matching circuits to match low frequency transmitters to their antennas.
  Another method to control the inductance without any moving parts requires an additional DC current bias winding which controls the permeability of an easily saturable core material. See Magnetic amplifier.
caption: A "roller coil", an adjustable air-core RF inductor used in the tuned circuits of radio transmitters. One of the contacts to the coil is made by the small grooved wheel, which rides on the wire. Turning the shaft rotates the coil, moving the contact wheel up or down the coil, allowing more or fewer turns of the coil into the circuit, to change the inductance.
exif: {}