up148 wrote:That is pretty amazing when you think about it. It looks like old design, but maybe not. Very interesting that it is/was used to calibrate volt meters of later design.
History of the Standard Cell
In 1893 the International Electrical Congress meeting in Chicago chose the Clark cell, a cell devised by Latimer Clark in 1872 as the standard of emf (the Volt) to which they assigned a value of 1.434 international volts at 15 °C. The Clark cell and this value for it were legalized as the standard of emf in the United States by an act of Congress, July 12, 1894.
During the years 1893 to 1905 the standard cell devised by Edward Weston was found to have many advantages over the Clark cell and was officially adopted in 1908 at the London International Conference on Electrical Units and Standards as the world standard for of emf. This London Conference went further and adopted provisionally 1.0184 V as the emf of the Weston Normal Cell at 20 °C. Later, after new Weston Normal Cells had settled in, the value of a Standard Cell was redefined as 1.018273 V.
At one time the best standards for voltage were electrochemical cells (Standard Cells). But no matter how carefully such cells were fabricated, their voltages inevitably drifted from month to month. In July 1972, the United States redefined its working volt in terms of the Josephson effect, predicted in 1962 by Brian Josephson, a graduate student at the University of Cambridge in the United Kingdom.
The Josephson Junction voltage standards are expensive and difficult to use. So, about the same time, Hewett Packard and Fluke came up with Zener Voltage references to replace the Standard Cell. They had 1, 1.018, 1.019 volt outputs (the Fluke also had a 10 volt output.)
A used Fluke voltage reference can be had for around $2,000 on eBay. If you just want a zener voltage reference to calibrate your own voltmeters, you can get one from Amazon, based on the AD584KH zener reference, for $22.96.
https://www.amazon.com/AD584KH-Precisio ... d_source=1