A few days later the triax cable arrived so I could give the ammeter a go... well, sort of. I needed something to connect to the end of the triax. This turned out to be a Trompeter cinch panel mount receptable - close up, a thing of beauty, as well it should be given how much it cost. This was fitted to a metal enclosure bought from a local electronics store, and because it's a picoammeter, a challenging resistor was mounted. The manual requires a range of resistors up to 100GOhm for performance testing, but just getting a 100Gohm resistor would cost me USD45 in postage alone, so I settled for a more reasonable 10GOhm, in the form of 2 x 5 GOhm 1% parts from Ohmite. These were mounted in the little metal box, being careful not to get any mucky finger prints on it.
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This was connected up between the ammeter input and voltage channel B, and the output button was pressed...
Yes! - the ammeter was working fine and the readings were once again pretty good, reading 9.94pA for 0.1V/10GOhm. Take that Uni-T! Interestingly, if the resistor enclosure was left open, just waving a hand near it was enough to send the reading all over the place.
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Of course, one of the next jobs will be to complete the set of test resistors...