File:Basketball variometer 1922.jpg

English: A variometer, a variable inductor used in radio receivers in the 1920s. This type was called a "basketball" variometer. It consists of two spherical coils of wire in series, one inside the other, the inner one mounted on a shaft so its axis be rotated with respect to the outer. The advantage of the variometer is that it allows inductance to be varied continuously from a maximum value to almost zero. When the two coils are collinear, with their magnetic fields in the same direction, the magnetic fields add, creating maximum inductance. When the shaft is turned so that the inner coil's axis is at an angle to the outer, the mutual inductance is less so the inductance is reduced. When the two coils are collinear with their magnetic fields in opposite directions, the fields cancel so the inductance is very small. The windings are wound in a "basket-weave" pattern so adjacent wires are not parallel, to reduce proximity effect losses and parasitic capacitance at high frequencies.