Weston, the Weston Cell, and the Volt

摘要

The measurement of electromotive force, potential, or voltage difference has been central to measurements ever since the concept of potential in electricity was first understood. The definition of the volt changed a few times throughout the course of history and at one point it was even based on electrochemistry, on the so-called Clark cell. The international volt was defined in 1893 as 1/1.434 electromotive force of the Clark cell. This definition lasted until 1908. The Clark cells used zinc or zinc amalgam for the anode and mercury in a saturated aqueous solution of zinc sulfate for a cathode, with a paste of mercurous sulfate as a depolarizer. The cell design had a drawback in a rather significant temperature coefficient and also suffered corrosion problems, which were caused by platinum wires that were alloying with the zinc amalgam in the glass envelope. In 1908 the definition based on the Clark cell was supplanted by a definition based on the international ohm and international ampere (through Ohm's law). Neither definition was of much practical use in helping to calibrate the precise Poggendorff compensation potentiometer. The standard for the use in the Poggendorff apparatus became a cell known as the Weston cell, named after its inventor Edward Weston, who came up with its design in 1893. This cell was adopted as the International Standard for EMF used from 1911 until 1990.