The vacuum pump
The invention of the air pump by Otto von Guericke led to big advances in the study of air pressure. Fig. 10.4 shows the principle of a modern type of pump. Each time the piston is at the bottom of the cylinder some air from the vessel to be evacuated expands into the space above the piston. On each upstroke the air above the piston is carried out through the valve A. The oil on top of the piston acts not only as a lubricant and air seal but also fills the dead space between piston and valve at the top of the stroke and ensures removal of all air trapped there.