Use the noun transformer when you're talking about a device that conducts electricity and controls the voltage that can travel between one circuit and another.
Electricians are familiar with transformers, since they are commonly used to manage the voltage of electricity that powers houses and some businesses. Sometimes you can see a transformer attached to a utility pole on a residential street. The word transformer was coined in 1883 from the French transformateur, which in turn comes from the Latin transformare, "change the shape or form of." The prefix trans, or "across or beyond," gives a clue to this meaning.