Restoring and testing a Pixii version of Coulomb’s Torsion Balance acquired by the University of Barcelona in 1848

By Júlia Garcia

This research was supported by a SIS Grant. A longer piece on this subject will feature in a future SIS Bulletin.

Figure 1. Pixii’s electrostatic torsion balance after its restoration in the History of Electromagnetism Exhibition. Credit: Facultat de Física de la Universitat de Barcelona.

In 1785 Charles-Augustin Coulomb read his first memoir on electricity and magnetism in the French Académie des Sciences, where he described a torsion balance he had invented, and which he used to obtain the law that would later bear his name. From that point forward the instrument’s presence became common in physics cabinets, even becoming mandatory in Spanish higher education institutions in the nineteenth century.

The instrument we examined is a Pixii torsion balance, most probably acquired by the University of Barcelona in September 1848 (Figure 1). We restored it and researched its biography so that it would shed light onto the characteristics of scientific education in Spain, specifically in the University of Barcelona, during the second half of the nineteenth century. The instrument, due to its materials and design, fails to replicate Coulomb’s experiment faithfully but is partially successful at representing it in a didactic manner.

To learn more about the material intricacies of Coulomb’s experiment, his experience is replicated as faithfully as possible, concluding that it is in no way trivial, as many physics manuals treat it. There has been active discussion during the past thirty years about the possibility that Coulomb may have reached the inverse-square relationship through theory rather than experimentation, because of how complicated it was – and still is – to replicate his experiment by operating an electrostatic torsion balance the way he described. Other considerations could explain the difference in results when seemingly following Coulomb’s instructions, both in the past and in the present. Back in the eighteenth century, the nature of electricity remained largely unknown, and could have contributed to misinterpretations of Coulomb’s memoir. Coulomb’s ample experience with wire and torsion experiments could have also been a major factor in helping him succeed where others failed.

In the forthcoming article in the SIS Bulletin, we look at an instrument’s past, not only through archival research but also through the replication of its use, so that it may tell us as much as it can about the context in which it was created and manufactured.

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