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.