One further remark: These high resolution, low noise DMMs, as 3458A, 8508A, 34420A all are capable to replace the 845A null voltmeter! All have higher sensitivity, and at least same or better noise performance. (see also the FLUKE AN) Especially in this application, i.e. low impedance sources, the bias current plays no role. But also in the usual, higher impedance applications, like Wheatstone Bridges, where the 845A was intended for, the modern instruments can also be used without any problem. Their bias currents are all specified to be < 20..50pA, and may be taken into account in the uncertainty budget. In practisey, my own 3458A shows < 2pA only. In contrast to that, the 845A was never (!) specified for bias current. Fluke only claimed something like > 10^12 Ohm when nulled to < 10µV, or so. The 845A contains an optical chopper, but I'm not convinced that this design really is superior to modern FET choppers, which can be found in the instruments mentioned. Maybe, volt-nuts owners of an 845A can really determine its leakage / bias current. Maybe also, that this mystified instrument really is not as good in this aspect, as everybody assumed for the past decades. Frank
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