Apex locators are electronic instruments used in endodontics that
measure the impedance, frequency and resistance of the surrounding material in order to locate the working length of the root canal to be endodontized.
The principle is
based on the electrical resistance of different tissues. When the circuit is complete, resistance decreases and current begins to flow. Apex locators function by using the human body to complete an electrical circuit. One side of the apex locator's circuitry is connected to an endodontic instrument.