reostat

noun3

1. •
   An electrical resistor with two terminals, the resistance of which can be varied continuously by moving a knob or slider to regulate the flow of an electric current.
2. •
   Such a device used to regulate the brightness of an electric light; a dimmer.

   • The light in the radio cabin was very dim, the rheostat of the angled table lamp had been turned down until only a faint pool of yellow fell on the scratched metal of the table, […] — 1966
3. •
   (figurative) A notional device which causes some quality to vary.

   • The world had dimmed a little when she was gone, as if there were a rheostat on the sun and somebody had turned it down, just one notch. — 1968