Article ID: CBB001202178

The Daniell cell, Ohm's law, and the emergence of the International System of Units (2014)


Telegraphy originated in the 1830s and 40 s 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, but each telegraphy 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 109 emu units of resistance and the volt as 108 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). As it was, 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.

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Authors & Contributors
Morris-Reich, Amos
Mertens, Joost
Onishi, Hiroshi
Mills, Allan A.
Rossi, Arcangelo
D'Agostino, Salvatore
Physis: Rivista Internazionale di Storia della Scienza
British Journal for the History of Science
Archives Internationales d'Histoire des Sciences
化学史研究 [Kagakushi kenkyū; Journal of the Japanese Society for the History of Chemistry]
Annals of Science: The History of Science and Technology
History and Technology
University Press of Kansas
Oxford University Press
Cambridge University Press
Standardization and standards
Electricity; magnetism
Giorgi, Giovanni
Leclanché, Georges Lionel
Faraday, Michael
Davy, Humphry
Majorana, Quirino
Hansteen, Christopher
Time Periods
19th century
20th century, early
20th century, late
21st century
20th century
Pacific Northwest (North America)
United States
International Bureau of Weights and Measures
Freie Universität Berlin

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