- 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. - Correct determination of tooth length is a crucial factor for the success of endodontic therapy. *Note that extra delivery fees may be applied to this item. For more info contact Toothpick Customer Support or check out Terms and Conditions policy.