Persistence Of A Filament
To demonstrate that high-frequency AC looks like DC.
Audio oscillator, 100 watt amplifier, and sealed beam auto headlamp.
A sealed beam car headlamp is run by an oscillator and audio amplifier. Below about 20 Hz the light flickers, but above 20 Hz it appears continuous. This persistence is caused partly by your eye and partly by the heating of the filament remaining relatively constant over the period of the applied AC voltage.