WSI offers several standard supplies to cover all requirements. The magnetic field generated by a precision electromagnet system has several characteristics which should be considered in the final selection. Stability of the field means it is independent of changes in line voltage, load impedance, noise, temperature drift or the introduction of extraneous magnetic fields. Homogeneity is usually expressed as a fraction of the total field. The magnetic intensity nearly always decreases slightly off of the center point of the poles, but plots such as the ones shown indicate the homogeneity of the field.
Homogeneity is a function of air gap, pole face diameter and field intensity.
For example, homogeneity of the field may be improved dramatically by use of ring-shimmed pole caps. A micrometer head on each shim rod positioner provides quick and calibrated optimization of the shim position at any field strength. For air gaps greater than 2 inches, ring shims are often employed to increase field strength at the outer diameter of the pole caps. This compensates for the natural fall off in field strength and thereby improves homogeneity across the pole face. Properly designed ring shims can improve homogeneity at the field for which they were designed by about a factor of five.