Abstract: On the basis of the analytical result of Betz's ring by the approximate theory of magnetic dipole chains, the function below was derived for the magnetization of a testing ring through the threading-bar technique: (m-1)/m lnm=2(h-a)/Dln(D/D-2h)(in which, m=D/d, D-outer diameter of the ring, d-inner diameter of the ring, a-radius of the little hole, h-buried depth of the little hole). Whereby it is known that only three among D, d, a and h are independent. The digital calculation from this formula proved that the design of the British standard testing ring BS 6072—1981 was reasonable, and that the core diameters of the Japanese comparing blocks Type B, JISG 0565—1982 were too small to reach their employed target. The rational sizes of the inner diameters after innovation were given in the present paper.