Magnet having pole faces

摘要

The magnet includes two spaced-apart pole pieces with generally opposing pole faces. One pole face has an axis extending towards the other pole face. Circular and annular shim trays (68,72) are coaxially aligned with the axis and attached to the initial pole face. The shim trays have shims (70) each with a trapezoid shape when viewed facing the initial pole face. The trapezoid has straight shorter and longer bases and sides. The shorter base is radially disposed closer to the axis than is the longer base. The sides are aligned along radius lines drawn from the axis to the respective sides. The trapezoidal shaped shims may be arranged on the circumferences of imaginary concentric circles on the circular shim trays. Trapezoids on the same circumference have the same trapezoidal shape and trapezoids on different circumferences have different trapezoidal shapes.