THE MEASUREMENT OF ELECTRIC-FIELD, AND THE INFLUENCE OF SURFACE-CHARGE, IN MAGNETIC STIMULATION

A circular magnetic stimulator coil placed perpendicularly to the surface of a large uniform conductor induces surface charge. The resulting electrostatic field reduces the total electric field within the conductor to 58% of the value in the absence of surface charge. The properties of 3 kinds of probe for measuring the effect of a magnetic stimulator are considered. A short dipole electric field probe is the only one which correctly measures the total electric field, including the contribution from any surface charge. A search coil generally gives incorrect results, since it is insensitive to the electrostatic field.