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At the time of their introduction, HP voltmeters offered unprecedented reliability for their price.
The first voltmeter, the 400A, was introduced early in 1942 after a design by Dave Packard in 1941. It was manufactured until 1958. Notable for stability (it was one of the earliest vacuum tube voltmeters to need no initial adjustment for zero and none for drift) and for wide high input impedance over 1 MHz bandwidth, the 400A became an industry classic. A key to achieving its performance was application of negative feedback in amounts then unheard of.
This excerpt from the HP instrument catalog describes its features:
The 400B was based on a design originally made by Dave Packard in 1941.