[Skywave]

An extract from 'Radio Constructor', July 1972: a d.c. voltage calibrator.
I'm submitting this abridged article for a peer review: cct. is attached.

General concept.
The basic idea is that by comparing two d.c. voltages of the same polarity that appear across a load and adjusting one voltage so that the two voltages are equal, zero current flows into the load. If that load is a voltmeter, its calibration can then be checked with no current flowing in that voltmeter - so the voltmeter does not load the calibrating voltage source.

Circuit note.
The designer has chosen to to use a start-up circuit to get sufficient voltage to initiate the firing of the two stabilizer valves, which once they have fired, the cct. then returns to its 'normal' configuration. The only reason he has chosen that arrangement is simply because his voltage source - the half-wave rect. plus the voltage from the transformer - is insufficient to trigger the starting voltage of the two stabilizer valves if the string of load resistors are then still in circuit.

There are various other comments I could add, but all of the above will suffice by way of an introduction.

► Your thoughts and comments on this design are cordially invited.

Al.

Edit. Sorry about the small size of the cct. I couldn't find a way of making it appear larger here.