Building a better Guinea and Feather

Step 15 Vacuum pump, gauge, and cart

The vacuum gauge was purchased off eBay. Standard atmospheric pressure is 76 cm of Hg (or 760 mmHg which is also called 760 Torr), and the gauge reads the pressure relative to atmosphere. For example, if the gauge reads -70 cm Hg, that's 76 - 70 cm Hg = 6 cm Hg (or 60 Torr). Some mechanical pumps can get down to a few millitorr, which is more than sufficient to remove the air resistance in this experiment.