Abstract: Apparatus for the determination and the digital display of the position in the tooth canal of a root treating device, comprises a saliva suction device in circuit with the root treating device to sense the variation of the impedance between the gum and the root treating device. That impedance varies inversely as the depth of the root treating device; and so the variations in impedance are amplified and converted into DC voltage levels which are applied to a series of light displays in such a way that one light display will be most brightly illuminated while its neighbors will be less brightly illuminated, the comparative illuminations of the adjacent light displays giving a quick visual indication of the position of the root treating device in the tooth canal. An audible signal can also be provided.