Helmholtz coils for calibration of probes and sensors: limits of magnetic field accuracy and uniformity

Helmholtz coils have been around us for years, but few people now seem to understand their capabilities. The paper explains the accuracy and field uniformity limits of helmholtz coils for use as a calibration standard for magnetic field probes and sensors. The magnetic field generation accuracy depends on the accuracy with which helmholtz coils are constructed and the accuracy with which the current through them is maintained. The user is shown how to determine the accuracy of the generated magnetic field based on physical measurements of distance or spacing between the coils, their radii, the thicknesses of the windings on each coil and number of turns. Ways to estimate the maximum usable frequency and minimum space needed around them are given. Procedures are given so that the user may determine the field uniformity versus volume around the center point of the set of coils, and thus determine the coil characteristics needed to calibrate a particular probe or sensor