Abstract: To determine the relative positions of two bodies, an alternating current is passed through a conductive path in the first body, establishing an alternating magnetic field around the body, the amplitude of the field decreasing in inverse proportion to the distance from the conductive path. Coil pairs are supported on a second body so as to essentially surround the first body, the coils being used to produce voltages proportional to the intensity of the magnetic field, thereby providing differential distance information. The voltage differences of the coils can be used for establishing the shape of the first body or for positioning the second body to a coaxial location of the coil pairs with respect to the first body. The determination is largely free from interference.