Conductivity differences distort probe measurements of magnetically induced electric fields

Measuring internal induced electric fields in animals with a miniaturized probe involves a potential error related to the difference between the hole conductivity (σ h ) and the surrounding tissue conductivity (σ t ). Theory was developed to describe this phenomenon and checked by probe measurements in agar‐filled petri dishes. The value measured in the hole is 2σ t /(σ h + σ t ) times the actual field in the tissue. For example, a probe hole in muscle, which is filled with blood, could yield a measurement only about 22% of the true value in the muscle. This potential source of error can be mitigated to some extent by not actually cutting a hole, by using a low‐conductivity (e.g., 0.2 S/m) coupling medium in the hole, or by ensuring contact between the probe's electrodes and the tissue. © 1994 Wiley‐Liss, Inc.