variable-pitch propeller. A propeller whose pitch can be changed in flight. A two-position propeller is one that can be set to a low pitch angle for takeoff, and then in flight, the pitch angle can be increased for a more economical cruise.
A constant-speed propeller is a variable-pitch propeller whose pitch is controlled by a governor to keep the RPM of the engine constant as the air loads on the propeller change.
variable resistor (electrical component). A resistor whose resistance can be changed by varying the length of the resistance element in the circuit. The most commonly used variable resistors are rheostats and potentiometers. Rheostats have two terminals, one at the end of the resistance element and one at the wiper, or movable contact. Potentiometers have three terminals, one at either end of the resistance element and one at the wiper.
variable restrictor (fluid flow control device). An adjustable device in a fluid power system. A variable restrictor is usually some form of variable orifice that controls the amount of fluid allowed to flow in a given time when it is acted on by a specific pressure.
Variac. The registered trade name for a toroidal-wound autotransformer. A portion of the winding is connected to the power line, and it serves as the primary of the transformer. A carbon brush riding on a bare portion of the winding is moved by the control knob. The position of this brush determines the number of turns of wire used as the secondary winding.
A Variac can vary the secondary voltage from zero to approximately 117% of the primary voltage.

variation (magnetic compass error). The compass error caused by the difference in the physical locations of the magnetic north pole, the location to which the magnetic compass points, and the geographic north pole, the reference from which navigational charts are drawn.
Variation is shown on a chart by isogonic lines, along which the variation is a constant number of degrees. For land navigation, variation is called declination.
varistor (electronic component). A special type of semiconductor device whose resistance varies as the voltage across it changes. A varistor is sometimes called a voltage-dependent resistor (VDR).
varmeter (electrical measuring instrument). An electrical instrument used to measure the reactive power in an alternating current circuit. Reactive power is the power caused by the portion of the current not in phase with the voltage. Reactive power is measured in volt-amps reactive (VARs) or kilovolt-amps reactive (KVARs).