Something that has always puzzled me is how we know the accuracy of voltmeters. From googling around, it seems that today's digital voltmeters use an ADC. An ADC works by comparing the voltage that is to be measured against a highly precise known voltage reference. However, how is the voltage of that highly precise voltage reference known? It seems to be like a chicken vs. egg kind of problem - knowing the accuracy of the voltmeter ADC reference voltage relies on the accuracy of another voltmeter using another reference voltage which relies on the accuracy of another voltmeter using another reference voltage and so on and it continues infinitely.

From looking at the Wikipedia page for a voltmeter, it seems like something such as a Weston cell, which uses a reproducible and stable chemical reaction, is used as the ultimate reference voltage for calibration. But still, that falls prey to the chicken vs. egg dilemma - how do we know the voltage of the Weston cell without again using a voltmeter?