Application of atmospheric and liquid pressure
The rubber sucker
A very useful application of atmospheric pressure is found in a circular shallow rubber cup known as a sucker. These are used for attaching notices to shop windows and for similar purposes. When the rubber is moistened to obtain a good air seal and pressed on a smooth flat surface the cup is flattened and air squeezed out from beneath it. Atmospheric pressure then holds the sucker firmly on to the surface.
The discs of thin plastic material used for attaching licences to motor-car windscreens are based on the same principle. Large rubber suckers are used in industry. Fig. 11.1 shows a lifting beam with two suckers each capable of supporting 500 kg each. The weight of the beam expels the air from the suckers as they are lowered on to the aluminium sheet, and when the
sheet has been lifted the suction is released by admitting air through a lever-operated valve.