Megger, which is a brand name for what's really an insulation tester, as oft used by Electricians and Electrical Engineers. We bought this one in the early to mid 1990s... and I remember ordering it. It's finally given up the ghost. Thought I'd managed to repair it, but no, so time to seek a replacement. Over 25 years use wasn't bad! We have a less old version which only tests up to 500V instead of the 1000V of the above. That'll serve as a temporary replacement for some classes and testing lower voltage (e.g. household mains) systems.