Abstract: On the basis of the analytical result of Betz's ring by the approximate theory of magnetic dipole chains.Whereby it is known that among D,d,a and h,only three of them are independent.The digital calculation from this formula proved that the design of the British standard testing ring BS 6072-81 was reasonable,and that the core diameters of the Japanese comparing blocks Type B,JISG 0565-82 were too small to reach their employed target.The rational sizes of the inside diameters after innovation were given in the present paper.