Electric resistance between oral mucosa and PDL is always constant that is:

FIRST-GENERATION APEX LOCATORS (RESISTANCE APEX LOCATORS) They are also known as resistance apex locators that measure opposition to the flow of direct current, that is, resistance. It is based on the principle that resistance offered by periodontal ligament and oral mucous membrane is the same, that is, 6.5 K ohms. Initially, Sono-Explorer was imported from Japan by Amadent, (Port Jefferson, New York) but nowadays first generation apex locators are off the practice. Blood, pus, chelating agents, irrigants, and other materials used within the canal can give false readings.