The Museum of T&M Instruments

"The Keithley Model 662 Guarded dc Differential Voltmeter is an extremely precise self-contained, guarded potentiometer. It measures from 100 millivolts to 500 volts with 0.01% limit of error, and below 100 millivolts within 10 microvolts. Since the limit of error includes all stability considerations, the unit need not be periodically calibrated or manually restandardized for a full year. The 500-volt reference supply permits infinite impedance at null for measurements from 5 to 500 volts. Six dials assure that every voltage setting has at least 5-dial resolution." (Keithley, Manual 662, 1963)

BRIEF SPECS

Range	0.5 volts to 500 volts f.s.
Accuracy	± 0.01% or reading or 10 microvolts
Output	10 to 25 millivolts f.s.
Power	50 watts
Dimensions	5-1/2" x 17-1/2" x 13-1/2"
Weight	25 lbs (12kg)
Technology	tube
Price	approx. $ 575 (model 660 in 1964)

The term 'differential voltmeter' is still occupied by J. Fluke Co. Other companies like Hewlett-Packard (Model 740A) and Keithley with its model 662 shown here did not danger Fluke's position (other than the dvm). The Keithley instrument is very sturdily built and all used materials and components are really high quality. The Kelvin-Varley-Divider in the instrument is supplied by a very stable 500 VDC regulated source. An additional 1.34V mercury cell (type PX13T2) plus pot is used for compensate any unwanted thermal emf up to ±30 uV. Mercury cells are not longer manufactured due to waste problems. The good news is that these cells are ultra stable and longlife if you are able to grab one. There is still demand and offerings from the photographer's corner, as they are used in older light measuring devices; one possible replacement is the Varta V625PX.
Keithley still is one of the few remaining manufacturers of high-end multimeters and special voltmeters (see actual 2000 series). Look at their internet homepage - you can still download complete manuals for ther 1970 era instruments - wow!