Accurate as hell, very high input resistance. Range that makes conventional VTVMs blush.
They got four light bulbs in there and a wheel that spins. I imagine it is PWM actually, what else ? But how the hell do they get the accuracy ? That old thing agrees with a Fluke 8846 (I think that's the model) which, while not a Keithley electrometer is pretty damn good.
Sometimes I am amazed by old technology. Even things before I was born are seriously accurate and versatile. Have a look at y old 561A Tek scope. Even though the bandwidth is low the thing is accurate as hell and has dual tim ebase. In what, 1957 ?
And this 412a can read 450 megohms accurately, which was proven literally b y 45, precision 10 meg resistors in series, each of which was measured sepa rately before being strung together for the purpose.
Not sure right now exactly when the 412a was made, but I think it predates me.
How did they get it so accurate ?