"The numbers [on the amplifier] all go to eleven.... Well, it's one louder, isn't it? It's not ten."
This Is Spinal Tap (1984)
A variable resistor—or rheostat—is a device that controls the flow of a current, rheo being Greek for "to flow." Rheostats are employed to adjust the current in electric machines, and to vary the resistance in electric circuits. Examples of their use are the dimming of lights and the controlling of a motor's speed. However, the way that most people encounter variable resistors is behind the knobs on radios or under the sliders on more complicated audio equipment. Turning the volume knob, you are moving the 'finger' of a variable resistor, changing the tapping point and therefore the' resistance, supplying more or less power to the speakers, which makes the sound louder or quieter.
The variable resistor's design is based on the Wheatstone Bridge, which was invented by British mathematician Samuel Christie (1784-1865) but named after his compatriot Charles Wheatstone (1802-1875), the scientist who popularized it. It could measure electrical resistance by using four resistors, a battery, and a galvanometer (an instrument for detecting electric current and measuring it). From this starting point, Wheatstone devised the rheostat, which was able to measure (and control) an unknown resistance when being placed in series with a rheoscope (a device to measure electric current) and a rheomotor (a source of electric current). You had to record the reading of the rheoscope before inserting the rheostat into the circuit as a subsitute for the unknown resistance. The rheostat was then set to give the same current reading on the rheoscope as before.
Rheostats thus allow you to simulate the resistances of long cables without having to use them.
