Ohmmeter 'Megger' tester, c 1950s.
© Science Museum / Science & Society Picture Library
Ohmmeters are direct-reading instruments for measuring the electrical resistance of a circuit or component in ohms. A megohmmeter, commonly known as a 'megger', is basically a similar instrument but is able to apply a much higher value of dc (direct current) voltage to the item being tested than an ordinary ohmmeter. The majority of substances lacking mobile electrons, generally non-metals, have high resistivity and make good insulators.