Abstract: A device for and method of sensing conditions in a well bore by positioning a radioactive source adjacent the surface of the well bore for emitting a radioactive signal across the well bore. A means for detecting the radioactive signal is positioned opposite the radioactive source and electrical means are mounted with the detecting means for electrically measuring the intensity and frequency of occurrence of each detected signal. An electrical signal is provided by the electrical means for indicating the presence or absence of drilling fluids and/or the density of such fluids at the surface of the well bore.