Abstract: The digital apical foramen locating apparatus of the present invention utilizes the recognized fact that the conductance from the apical foramen to a patient's lip is not only equal to the conductance from the sulcus to the lip but is also constant from patient to patient. It further utilizes the recognition that there is a linear correspondence in the change of the conductance as the apical foramen is approached in root canal therapy. Utilizing this knowledge, the present invention employs a pair of fixed resistors to calibrate a single oscillator. This same oscillator is then used in the actual measurement and a comparison is made between the measured values and the stored reference values. In addition to a digital readout in 0.1 mm increments, there is an audible display whose cadence increases as the apex is approached and a graphical display which graphically illustrates to the dentist the location of the reamer in the canal and also the status of the probe's reliability.

Abstract: A numeric readout device is provided to display the distance of an instrument in a tooth from the peridontal membrane surrounding the tooth, including a pair of oscillator circuits with a numerical relationship determinator reading a distance measurement depended upon the impedance between the instrument and the gingival sulcus.