potentiometer n.

late 14c., "possible" (as opposed to actual), "capable of being or becoming," from Old French potenciel and directly from Medieval Latin potentialis "potential," from Latin potentia "power, might, force;" figuratively "political power, authority, influence," from potens "powerful," from potis "powerful, able, capable; possible;" of persons, "better, preferable; chief, principal; strongest, foremost," from PIE root *poti- "powerful; lord."

The noun, meaning "that which is possible, anything that may be" is attested by 1817 (Coleridge), from the adjective. Middle English had potencies (plural) "a caustic medicine" (early 15c.).

word-forming element meaning "device or instrument for measuring;" commonly -ometer, occasionally -imeter; from French -mètre, from Greek metron "a measure," from PIE root *me- (2) "to measure."

Proto-Indo-European root meaning "powerful; lord." 

It forms all or part of: bashaw; compos mentis; despot; hospodar; host (n.1) "person who receives guests;" idempotent; impotent; omnipotent; pasha; plenipotentiary; posse; possess; possible; potence; potency; potent; potentate; potential; potentiate; potentiometer; power; totipotent. 

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit patih "master, husband;" Greek posis, Lithuanian patis "husband;" Latin potis "powerful, able, capable; possible."