The second generation
Second generation apex locators were of the singlefrequency
impedance type which used impedance
measurements instead of resistance to measure location
within the canal. Impedance is comprised of resistance
and capacitance and has a sinusoidal amplitude trace.
The property is utilized to measure distance in different
canal conditions by using different frequencies.
The change in frequency method of measuring was
developed by Inoue in 1971 as the Sono-Explorer
(Hayashi Dental Supply, Tokyo, Japan) which calibrated
at the periodontal pocket of each tooth and
measured by the feedback of the oscillator loop (Inoue
1972). The beeping of the device indicated when the
apex was reached, so some clinicians erroneously
thought that it measured by using sound waves (Inoue
1973). A later model, the Sono-Explorer Mk III uses a
meter to indicate distance to apex (Inoue & Skinner
1985).