Characterizing and calibrating a large Helmholtz coil at low ac magnetic field levels with peak magnitudes below the earth's magnetic field

Characterizing and calibrating a low impedance large Helmholtz coil generat ing 60 Hz magnetic fields with amplitudes well below the earth's magnetic f ield is difficult and imprecise when coil shielding is not available and no ise is an issue. Parameters influencing the calibration process such as tem perature and coil impedance need to be figured in the calibration process. A simple and reliable calibration technique is developed and used to measur e low amplitude fields over a spatial grid using a standard Hall effect pro be gaussmeter. These low amplitude fields are typically hard or impossible to detect in the presence of background fields when using the gaussmeter in the conventional manner. Standard deviations of two milligauss and less ha ve been achieved over a spatial grid in a uniform field region. Theoretical and measured fields are compared yielding reasonable agreement for a large coil system designed and built for bioelectromagnetic experiments at the U niversity of Nevada at Las Vegas using simple tools. Theoretical results ne ed to be compared with and adjusted in accord with measurements taken over a large parameter space within the design constraints of the coil. Magnetic field measurements made over a four year period are shown to be consistent . Characterizing and calibrating large Helmholtz coils can be performed wit h rulers, levels, plumb lines, and inexpensive gaussmeters. (C) 2001 Americ an Institute of Physics.