Teardown of the Keithley 515A Megohm Wheatstone bridge, plus a tutorial on how Wheatsone bridges work and their applications.
Manual & Schematic:
https://download.tek.com/manual/515A(Model515A).pdf1960's Daven precision resistor and attenuator catalog:
http://www.technicalaudio.com/pdf/Daven/Daven_attenuator_switch_lab_Catalog.pdfKeithley electrometer teardown:
00:00 Keithley 515A Megohm Wheatstone bridge
03:50 - The Magic smoke test
04:00 - Zero check test
04:40 - Standardise calibration step
05:30 - Range calibration
06:21 - Measuring a 200M resistor
09:41 - Confirmation with a Keithley DMM7510 7 1/2 digit multimeter
10:37 - Weatstrong Bridge Tutorial
14:55 - Gaurd traces for high impedance measurement
16:22 - Teardown
28:25 - OUCH!