1843, "instrument for regulating or adjusting the resistance in a circuit," coined by English inventor Charles Wheatstone from Greek rheos "a flowing, stream" (see rheo-) + -stat "regulating device." Related: Rheostatic.
word-forming element meaning "current of a stream," but from late 19c. typically in reference to the flow or adjustment of electric current, from Greek rheos "a flowing, stream, current," which is related to rhein "to flow," rhythmos "rhythm" (from PIE root *sreu- "to flow").
word-forming element used in making names of devices for stabilizing or regulating (such as thermostat), from Greek statos "standing, stationary," from PIE *ste-to-, suffixed form of root *sta- "to stand, make or be firm." First used in heliostat "an instrument for causing the sun to appear stationary" (1742). Related: -static.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sravati "flows," srotah "stream;" Avestan thraotah- "stream, river," Old Persian rauta "river;" Greek rhein "to flow," rheos "a flowing, stream," rhythmos "rhythm," rhytos "fluid, liquid;" Old Irish sruaim, Irish sruth "stream, river;" Welsh ffrwd "stream;" Old Norse straumr, Old English stream; Lettish strauma "stream, river;" Lithuanian sravėti "to trickle, ooze;" Old Church Slavonic struja "river," o-strovu "island," literally "that which is surrounded by a river;" Polish strumień "brook."
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<a href="https://www.etymonline.com/word/rheostat">Etymology of rheostat by etymonline</a>
Harper, D. (n.d.). Etymology of rheostat. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/rheostat