An apex locator is an electronic device used in endodontics to determine the length of the root canal space. The electronic principle is relatively simple and is based on electrical resistance; when a circuit is complete (tissue is contacted by the tip of the file), resistance decreases markedly and current suddenly begins to flow. According to the device, this event is signaled by a beep, a buzz, a flashing light, digital readouts, or a pointer on a dial. The original electronic apex locators operated on the direct current principle. A problem with these devices was that conductive fluids such as hemorrhage, exudate, or irrigant in the canal would permit current flow and therefore a false reading. Newer devices are impedance-based, using alternating current of two frequencies;these measure and compare two electrical impedances that change as the file moves apically. The benefit is that these devices are much less affected by fluid conductive media in the canal. The impedancetype apex locators have been demonstrated to be 80 to 95% accurate in identifying the apical foramen. Therefore after obtaining a reading, 1 to 2 mm is subtracted as the corrected working length. Although there are some variations with newer devices, most operate similarly. One electrode is attached to the patient (commonly a lip clip); the other electrode is clipped to the file. The patient therefore forms part of the circuit. When current flows, the operator is notified by one of the aforementioned signals. After the length adjustment is made, a confirmatory radiograph is made (angled when indicated) with an appropriate size file at this length.