The name given to the instrument comes from the fact that it is made up of two hollow brass hemispheres and was designed by the wise burgomaster of Magdeburg, Otto von Guericke (1602-1686).
In 1650 he had invented and constructed the first pneumatic machine: a piston pump (suction and compression) quite similar to the one used with liquids which, with two valves, drew a part of the air contained in the recipient in which the vacuum was to be created into the pump body with every suction and eliminated it with the subsequent compression.
Using that machine, von Guericke showed the properties of elasticity, weight and pressure of air.
With the two hemispheres he performed spectacular experiments of great effectiveness and demonstrated atmospheric pressure.
The two hemispheres in the collection are made of brass, 11 cm in diameter and were probably part of the first endowment of instruments and machines for the Physics Laboratory sent to Father Cossu by Charles Emmanuel III (in the note countersigned by Father Cossu there were also two brass hemispheres).