A quantitative comparison is made between the results of the perturbation-theory analysis and experimental observations. It has been necessary to consider only the fundamental mode of propagation in a waveguide containing an equivalent dielectric slab and to use as the perturbation the change in the diagonal component of the ferrite permeability tensor as the ferrite becomes magnetized. An explicit expression for the phase shift is derived. A simplified form valid for ferrite specimens large enough to be useful in the reciprocal phase-shifter reveals the principles of operation. This result shows that dielectric concentration of the microwave fields within the ferrite produces a magnification of the permeability change as the ferrite specimen becomes magnetized in the small applied d.c. field.