TrainerNaive t1_j2r5rl7 wrote

Your house would soon be on fire if you fitted a simple rheostat in a regular wall box and used it to control more than a few watts.

The other folks are right, the earliest domestic lighting dimmers (late 60s onward) used semiconductor switches. Commercial premises, school halls and theatres had rheostat dimmers before that but they were in huge cabinets and dissipated huge amounts of heat.

Very low power fans might have been controlled by rheostat. More likely rotary variacs (variable auto transformers).

I am a chartered electrical and electronics engineer with over 50 years' experience.