Electric resistance between oral mucosa and PDL is always constant that is:
4.5 K ohm
5.5 K ohm
6.5 K ohm
7.5 K ohm
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.