MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
A balloon, with most of the air removed is clamped and placed in a vacuum tank. The air on the inside of the balloon is pushing out and air on the outside is pushing in. But, when some of the air is pumped out of the tank, the remaining air molecules do not push as hard on the balloon but the air inside is still pushing out at full strength. As more and more air is removed from the tank, the air in the balloon expands. The gauge measures the air pressure in hectopascals.
Model release not required. Property release not required.