The function of a transformer is easily understood and is simply the passage of electrical energy from one circuit to another by means of electromagnetic induction. This almost invariably involves windings around a core. The most usual use of a transformer is to transform one AC voltage to another by having primary and secondary windings of different lengths. Various different types of transformer exist but one of the most efficient employs a toroidal (doughnut shaped) core with concentric primary and secondary windings. This design lends itself to use as a variable transformer by having one of the secondary terminals attached to a sliding contact, running along an exposed track of the secondary windings. The whole device is often enclosed within a metal case, with a knob affixed to the central, rotating shaft. The knob sometimes turns on a graduated scale, but rather than reading volts (a voltmeter would be required), this is more usually a 0 - 100 or min-max scale. The variable transformer has many uses, with the voltage being infinitely variable between the maximum and minimum levels. For a fixed output voltage, a universal transformer is usually more suitable and some have a series of secondary take-offs, allowing for a stepped range of outputs.