The apparatus consists of a clear Perspex tube 380mm long x 50mm outside diameter provided with a light, freely sliding expanded polystyrene piston and a loose fitting metal cap. 

The tube is mounted by means of a bayonet-type fitting in a metal cap, upon a polished wood motor housing 170 x 170 x 130 mm high. A low voltage D.C motor is contained in the housing and is coupled to a crank driven agitator. The push red of the agitator carries a light plastic piston which oscillates in the lower end of the tube