The present disclosure describes an instrument for measuring the dielectric properties of biological tissue. The instrument includes a top electrode assembly and a bottom electrode assembly, the top electrode assembly including a top electrode and at least one shaft adjustably positionable to move the top electrode relative to the bottom electrode assembly, the bottom electrode assembly including a bottom test plate and a bottom electrode. The instrument also includes a testing cylinder coupled to the shaft and having an inner cavity defined therein that houses the top electrode and which is designed to enclose the bottom electrode therein. The testing cylinder is configured to reduce at least one of electric current, magnetic current, stray radiative RF fields and external capacitive leakage currents during activation of the top and bottom electrodes.