it measures resistance.
Usually a megaohmeter (or megger) is used to test high current feeder wires for ground faults before they are landed on a transformer.
An Ohmmeter is a device that is used for the measurement of Electrical Resistance. An ideal Ohmmeter consists of an electronic circuit having a constant current [I] passing through a resistance [R] & another circuit that measures the voltage[V]. Thus from Ohm's Law R= V/I. Micrommeter(or) Microohmmeter : To measure Smaller Resistances. Megohmeters (or) Megaohmeter (or) Megger : To measure Larger Resistances. e.g. Insulation Resistance of Electrical Cables.