A megohmmeter, also called a megger, is used to measure the extremely high electrical resistances found in machinery, such as an AC unit or electric motor. Megohmmeters work by generating a high-voltage, low-amperage current and passing it across the resistors.
Resistors are measured in ohms, and megohmmeters are used on resistors that have over 1,000,000 ohms. Megohmmeters are usually powered by batteries or hand-cranked. Despite the low amperage generated, a megohmmeter can create enough voltage to fry circuitry not rated for its output. It is important to always study the electrical diagram before using one. Also, users should never hook one up to a doorknob and crank it until it hums, no matter how tempting it may be.