The apparent resistance of a finite-thickness layer measured with a four-el
ectrode plunge probe depends on the electrode insertion depth, electrode sp
acing, and layer thickness, as well as the resistivity ratio of an underlyi
ng layer A physical model consisting of air, a saline solution layer, and a
n agar layer simulates the real situation of resistivity measurement. The s
aline layer represents the finite-thickness layer whose resistivity is to b
e measured by a plunge electrode probe, and the agar layer represents an un
derlying perturbing layer. A micropositioner controls the insertion depth o
f the four electrodes into the saline solution. With the apparent resistanc
e measured on a semi-infinite-thickness layer of saline solution as standar
d, measurement results show decreasing apparent resistance and increasing e
rror with increasing electrode insertion depth. This information is importa
nt for correct measurement of myocardial resistivity in vivo and in vitro.