Abstract: Basing on a problem in The Feynman Lectures （Vol. 2）, it is proved that the magnetic flux density along the axis of the ellipsoid of revolution which has coil on its surface is a constant under the condition which Feynman provided. Then it turned out that this case is suitable for all the quadric surfaces of revolution. At last a general formula is obtained applicable to them all.

Key words: magnetic field, quadric surface of revolution, symmetrical axis of revolution