In Chap. 5, the steady-state solution for the intensity in the ammonia beam maser was determined by energy conservation, that is, the energy given up by the molecules (gain) was equated to the losses in the maser cavity. Although attractive in its simplicity, this method provides no clue to the time depend­ ence of the intensity or to mode pulling (index of refraction) and cannot be used for treating multimode operation. These and other limitations can be avoided by describing maser action by a classical electromagnetic field gov­ erned by Maxwell’s equations and by demanding that the field be self-con­ sistent, that is, that the field E inducing polarization of the active medium be equal to the resulting field E ' as depicted in Fig. 8-1.