There are nice pdfs on the nist site. Voltage is a dervivated standard but through modern technic a very known one. In the beginning they useds cgs units. centimeter, gram, seconds ( if i recall this right) all others were derivated by that and the terms like Volt were in the same order as feet, tumb ect. They were workman units. 1 Volt was the value of a standard cell. Lord Kelvin ( thompson) was te first to measure the real volt ( as function of resustance and current) and found out the standard cell was not 1V. The first meters were galvanometers. They meaure current but you can convert that to voltage using resistance. The first accurate real voltmeter ( 4 quadrant electrometer) was an invention of Kelvin.
For resistance they used length coupled to current through the resistance of some material. That is why we still know the name potentiometer. The resisance was measured in meter ( cm/mm) it was a long straight thing with a sliding contact and a scale in cm beside it. Like measuring frequency ( wavelength) with a slotted line.
For current they used a current balance but I still do not know how they work. It looks like some mechanical divise.
Just a sidestep: calibrating pur sang is only testing if it is still within specs. So adjusting withing specs is not allways included. What you want to know is the drift over time. If you calibrate your meter every year and 1V is 0.9V after one year, they adjust it to 1V again ect without telling it was 0.9V you think your meter is perfect. The next year the calibrate it and its 0.8V, they adjust it to 1V again. So while you think its perfect, it is far from ( but it can be dtill within specs, so technical it is also perfect)
That is why specs are so important and things like min 1% or best 0.2% specs from our Chinese friend tell you nothing. ( besides they do also not state referenced to what, it can be 1% to theire 5 % calibrtor ( rather extreme but to get the idea) but it can be even be 0.1 % to the prototype, or in theory because the used parts, those numbers are pure commercial) a good brand gives it per range, over time under notification of temp and humiity. Most times 24H, 3 months and a year.
But if it is new 1V and the report states after a year it is 0.99V, and next year 0.995 and the 3rd year it is still 0.995V ect you know it is now aged and stable over time so if you then adjust it to 1V and next year it is fi 0.997V you know that if you meaure something in the year between calibrations that the value will probbly be somewhere , with extra margin, between 1 and 0.995V.
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