This radiometer differs from the one acquired by the Physics Laboratory in 1887 not only in size (24 cm in height), but because its staff in open and terminates with a frosted truncated cone which allows it to be connected to a suction pump.
As a teaching aid it is complementary to the first one. It demonstrates that there must be the proper amount of the residual gas in the bulb: if the vacuum has not been created, or is insufficient, there is no rotation: the same occurs when the vacuum is exasperated.
Indeed, in the first case collisions between the molecules of the residual gas are quite frequent and there is no great difference in speed between the molecules on one side and those on the other of the vanes; if the vacuum is exasperated, collisions are too few to transfer to the vanes the amount of motion required to turn the rotor.