Thanks for posting the question. There aren't too many days that go by when there's not something new to learn on this forum. I join the webmaster in thanking WDurrance for the answer.
My dad had an independent garage in the 50's and 60's and that was the last time I remember evern thinking about one of those old spark plug testers. Some old ideas are timeless.
It's easy to see that WDurrance is a natural born tester/experimenter.
Thanks for the good info.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in