User Manual

Rheostat
See also: Liquid rheostat
Charles Wheatstone's 1843 rheostat with a metal and a wooden cylinder
Charles Wheatstone's 1843 rheostat with a moving whisker
The most common way to vary the resistance in a circuit is to use a rheostat. The
word rheostat was coined about 1845 by Sir Charles Wheatstone, from the
Greek ῥέος rheos meaning "stream", and -στάτης -states (from ἱστάναι histanai, " to set, to cause to
stand") meaning "setter, regulating device",
[6][7][8]
which is a two-terminal variable resistor. The term
"rheostat" is becoming obsolete,
[9]
with the general term "potentiometer" replacing it. For low-power
applications (less than about 1 watt) a three-terminal potentiometer is often used, with one terminal
unconnected or connected to the wiper.
Where the rheostat must be rated for higher power (more than about 1 watt), it may be built with a
resistance wire wound around a semicircular insulator, with the wiper sliding from one turn of the
wire to the next. Sometimes a rheostat is made from resistance wire wound on a heat-resisting
cylinder, with the slider made from a number of metal fingers that grip lightly onto a small portion of
the turns of resistance wire. The "fingers" can be moved along the coil of resistance wire by a sliding
knob thus changing the "tapping" point. Wire-wound rheostats made with ratings up to several
thousand watts are used in applications such as DC motor drives, electric welding controls, or in the
controls for generators. The rating of the rheostat is given with the full resistance value and the
allowable power dissipation is proportional to the fraction of the total device resistance in circuit.
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Electronic Symbol for rheostat