Weston Standard Cell

Every cell before this one was built to give current. This one was built to give a NUMBER, and it held that number so well that for most of the twentieth century the volt itself was defined by a shelf of these jars. The problem Edward Weston solved was not power, it was drift. A Daniell cell is steady enough to run a telegraph and useless as a reference: warm the room and its voltage moves, and you have no way to say by how much. The Clark cell that preceded Weston's was a genuine standard but changed 1.15 mV for every degree, so every reading needed a correction bigger than the precision anyone wanted. US 494,827 "Voltaic Cell", filed 24. November 1891 and granted 4. April 1893, is Weston's answer. One electrode is "an amalgam of cadmium and mercury"; the other is "pure mercury and proto-sulphate of mercury" - what we now write as mercurous sulphate, Hg2SO4. Between them sits a saturated solution of cadmium sulphate. The patent's own claim is not about voltage at all: it claims a cell "whose electromotive force is practically independent of temperature changes". It gives 1.018636 V at 20 degrees C and moves roughly 40 microvolts per degree - about 40 parts per million, some thirty times flatter than the Clark cell, which it replaced as the standard in 1905. Weston's own vessel is two cylindrical glass tubes joined by a cross tube, platinum wires sealed through the bottom of each, the whole thing bedded in a wooden block inside a brass case and filled with a beeswax, rosin and linseed oil compound - a jar built to be left alone. Its limit is the same as its virtue. Draw any real current and you move the number you came to read; the cell is a reference and never a supply. National laboratories retired it in 1990 for the Josephson effect, which is at least four orders of magnitude steadier and contains no mercury. Cadmium, mercury and mercurous sulphate are all toxic and this build is gated for that reason. Read the last two steps before the first: one of them tells you how to use a standard cell without destroying it, and the other tells you what to build instead if you only want the measurement.