The apparatus consists of a clear perspex tube provided with a light, freely sliding expanded polystyrene piston & a loose fitting metal cap. The tube is mounted by means of a bayonet type fitting in a cap, upon a housing. A low voltage DC motor is contained in the housing & is coupled to crank driven agitator. The push rod of the agitator carries a light plastic piston, which oscillates in the lower end of the tube. The apparatus is supplied complete with 3mm diameter metal spheres & four cardboard loading discs.