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in radio engineering, a variable-inductance coil designed for tuning oscillatory circuits. The type of variometer used in the 1920’s and 1930’s was divided into two sections connected in series: a fixed outer coil (the stator) and a movable inner coil (the rotor). The inductance of the variometer is equal to the sum of the inductances of the sections and the mutual inductance between them, which changes significantly when the rotor is turned 180°. This type of variometer was widely used to retune the frequency of the oscillatory circuit of a radio receiver. The variometer subsequently lost its original form and use and is now used in radio receivers as an inductance coil containing within its winding a core made of a magnetic dielectric that changes the induction within insignificant limits in order to adjust the frequency of the oscillatory circuit.