The Daniell Cell, Ohm's Law and the Emergence of the International System of Units

Abstract: Telegraphy originated in the 1830s and 40s and flourished in the following decades, but with a patchwork of electrical standards. Electromotive force was for the most part measured in units of the predominant Daniell cell. Each company had their own resistance standard. In 1862 the British Association for the Advancement of Science formed a committee to address this situation. By 1873 they had given definition to the electromagnetic system of units (emu) and defined the practical units of the ohm as ${10}^9$ emu units of resistance and the volt as ${10}^8$ emu units of electromotive force. These recommendations were ratified and expanded upon in a series of international congresses held between 1881 and 1904. A proposal by Giovanni Giorgi in 1901 took advantage of a coincidence between the conversion of the units of energy in the emu system (the erg) and in the practical system (the joule) in that the same conversion factor existed between the cgs based emu system and a theretofore undefined MKS system. By introducing another unit, X (where X could be any of the practical electrical units), Giorgi demonstrated that a self consistent MKSX system was tenable without the need for multiplying factors. Ultimately the ampere was selected as the fourth unit. It took nearly 60 years, but in 1960 Giorgi's proposal was incorporated as the core of the newly inaugurated International System of Units (SI). This article surveys the physics, physicists and events that contributed to those developments.