Screw the 25-watt bulb into the socket and connect to the Variac. Turn on the Variac to show that the bulb lights. Turn off the Variac and using the slip joint pliers, break the glass envelope off the bulb leaving the filament exposed. Fill the 1000 ml beaker with liquid nitrogen. Dip the filament into the liquid nitrogen so that it is completely immersed and turn on the Variac. The filament will glow and continue to do so until you take the filament out of the liquid nitrogen at which time it will burn out due to the rapid oxidation that take place in the oxygen-laden atmosphere.