PURPOSE: To show that helium gas contracts when it is cooled.

DESCRIPTION: A helium balloon which is cooled by a liquid nitrogen bath becomes heavier than air. When the balloon is removed from the liquid nitrogen it warms up, expands, and floats away, unless it is tethered as in the photograph.

SUGGESTIONS: A 7" diameter balloon works best; six inches is not enough to lift itself and eight inches cannot be cooled enough to prevent it from rising.

REFERENCES: (PIRA 4E10.20)

EQUIPMENT: Helium balloon, liquid nitrogen.

SETUP TIME: None.