  Models of the magnetic field enhancement at pits are discussed. In order to
build a model of pit, parameters that characterize a geometry of edges of pit,
such as a curvature radius and a slope angle of edge, should be included,
because the magnetic field is enhanced at an edge of a pit. A shape of the
bottom of the pit is not important, because the magnetic field attenuates at
the bottom. The simplest model of the pit is given by the two-dimensional pit
with a triangular section. The well-like pit, which is known to many
researchers, is a special case of this model. In this paper, idea and methods
to analytically evaluate the magnetic field enhancement factor of such models
are mentioned in detail. The well-like pit is considered as an instructive
exercise, where the famous results by Shemelin and Padamsee are reproduced
analytically. The triangular pit model, which is practically important for
studies of the quench of SRF cavity, are discussed in detail. Comparisons
between the prediction of the triangular-pit model and the vertical test
results are also shown.
