[en] FEDIFF is a finite-element circuit-network code in which conductors are modeled in a three-dimensional (3-D) lattice of metal plates, each plate possessing finite thickness. The plates are properly interconnected and magnetically coupled to all external and internal circuit elements. The lattice is transformed to become a circuit network and to be integrated. The result of computations is discussed, in which FEDIFF has been adapted to solve 3-D vertical field diffusion problems in the toroidal stabilizing shell of the ZT-40M device at Los Alamos, when a vertical magnetic field is applied as a pulse. The stabilizing shell is made of aluminum, 2.2 cm thick, that is one skin depth at 110 Hz. Because the shell is constructed with a poloidal insulated gap, the vertical magnetic field diffuses into the shell, differently in the gap region than elsewhere. With the existing aspect ratio of major to minor radius being six, we show tht field diffusion in a straight cylinder exhibits virtually the same characteristics as the toroidal configuration does, away from the gap, provided the cylinder possesses the same wall thickness and material makeup as the toroidal shell. The accuracy of the present eddy current field calculations is 2.8%