Reference: AP4
Category: Atmospheric Pressure
Purpose: Weighing air.
Description: A spherical flask of about 1 litre is suspended from one arm of a crude chemical balance it is counterpoised by weights in the other pan, the tap attached to the sphere being open. This gives (roughly) the true weight of the glass. Unhook the sphere from the stirrup , attach the rubber tube from a rotary vacuum pump to the glass tube and evacuate for around 30s. Close tap, and re-attach stirrup. There will now be an upthrust on the flask equal to the weight of air removed, and the weights in the other pan will have to be reduced by about 1.2g to restore the balance. Open the tap again: the air can be heard rushing into the flask (class must keep quiet), and the flask suddenly descends again.
