race 18 is held in contact with the rollers of the conical roller and cage assembly 15 by the magnetic attraction of the magnetic particles of the powder mixture to the magnets 16 by the lines of flux extending through the non magnetic base sheet 21 of the sample tray in the specific prototype arrangement employed the magnetic coupling force or attraction is about 50 kilograms as will be noted in fig3 the sample tray is retained in appropriate aligned position relative to the magnets 16 and in spaced clearance relationship therefrom enabling rotation as guided by the conical roller and cage assembly through the driving connection provided by the separable coupling 13 fig4 shows in detail a plan view of the arrangement of the magnets 16 on the magnet support plate 6 the basic magnets 16 are rectangular prisms which have been magnetized across their minor axis or shortest dimension to impart opposite polarity to their opposed major surfaces as shown in perspective in fig5 best results are obtained when an even number of magnets 16 are used the individual magnets 16 are positioned on the magnet support plate 6 in a circle which is concentric with the axis of rotation of the tray and are arranged such that their major axis extends radially outward from the centrally located drive shaft hole 10 it will be noted that a major edge of each magnet rather than the center line thereof is positioned along the radius of the circle of magnets thus each magnet 16 is.