During the 2023 workshop on “Material Culture in the History of Physics” held at the Deutsches Museum in Munich, a mystery object from the museum’s historical collection, which had been previously kept in the Bavarian Academy of Sciences (Bayerische Akademie der Wissenschaften) since at least the 1850s, if not earlier, was presented. The museum catalogue describes DM 1182 as a “Coulomb torsion balance with Jelin’s modifications” [1]; however, the apparatus displays significant visual differences from the well-known device created by Charles-Augustin de Coulomb to study electrical forces. Our team, consisting of Elisabetta Rossi, Daniel Liu, Jesse Garrison, and Julien Gressot, aimed to deepen the understanding of this peculiar object by uncovering the instrument’s history, exploring its function, and investigating modifications made by Julius Conrad von Yelin, a 19th-century German physicist.

Initial Encounter and Investigation

When we first saw the instrument, it was initially concealed under a cloth. At first glance, it appeared ordinary and incomplete, and not very “aesthetical”. Upon further examination, we recognized its complexity and potential significance. DM 1182 (Fig. 1) consists of a slightly irregular hexagonal wooden base with a rectangular hole (and a small door to close it); a round platform with a circular depression – matching the diameter of the glass dome – characterized by remarkably signs of use; a vertical wooden dowel (easily spinning) with a horizontal attached needle; a (removable) flat-convex brass instrument fitting into a transparent tube and connecting to a metal wire ending with a (screwing) ball coming out from the inner part of the basement; two vertical wooden columns, each with a kind of double hook facing down with very tiny holes; a wooden rectangular bridge connecting the two columns with a central knob.

Historical Context

We approached the investigation through an object biography framework, examining the instrument’s material construction, historical context, and intended function. This methodology allowed us to go beyond simple description and consider how DM 1182 was used and how it fit into the broader history of early scientific experimentation in electrostatics.
As already glimpsed, DM 1182 – which had been transferred to the Deutsches Museum in 1905, shortly before the museum opened to the public in 1906 – has been catalogued a Coulomb torsion balance with modifications.
In the 1780s years, the French physicist Charles-Augustin de Coulomb (1736-1806) used a suspended wire and a rotating rod (with an angular scale to measure displacement) to experimentally determine the law of attraction and repulsion between two electrical charges [2]. On the one hand, with respect to his original torsion balance (Fig. 2), DM 1182 lacks key features such as the wire, a suspended dumbbell, a counterweight for the rotating needle and a measurement scale – which, however, may have been lost during the displacement of the instrument. On the other hand, it shares similarities, such as the glass enclosure which shield the components from air currents while performing an electrical experiment.

Parallels and differences raise questions about the originality of the apparatus and whether (and which) parts were missing, modified or added later. The German physicist Julius Conrad Von Yelin (1771-1826), who, according to the catalogue, modified the apparatus, was a German physicist and mathematician involved in debates about electrical forces in the early 19th century. His name appears in the Bavarian Academy of Sciences’s 1850 catalog of the physical-mathematical collections for different instruments, one of which was used to investigate the repulsive forces of electricity by weight. [4]
Von Yelin modified several scientific instruments for experimental use. Currently dealing with his writings and books from the 1810s and 1820s, we believe that he adapted an earlier and perhaps simplified version of DM 1182, possibly a typical Columb balance, specifically to study repulsive electrical forces. His modifications may have been substantial – possibly, they may have involved the presence of the flat-convex brass conductor which carries a brass ball underneath the base, and the latter could be charged by a glass rod with a brass ball holder passing through the open rectangular door (we also thought there might be an electrophorus in the base, but there is no physical wooden support, i.e. the base is open, and we ruled this out).
In the early 1990s, Peter Heering replicated Coulomb’s experiment historically and discovered that, even with an instrument as delicate as Coulomb’s torsion balance, the precise measurements required to demonstrate the inverse quadratic relationship between charge and repulsive force were extraordinarily difficult, if not impossible [5-6]. DM 1182, with its relatively massive rotating dowel and the friction of its two mounting points, could not be sensitive enough for such a precise physical measurement.

Goals of Further Research

At present, we have evidence to state that DM 1182 is likely a variation of Coulomb’s original design, adapted for similar electrostatic studies by Von Yelin. However, the apparatus’ complete precise role and functioning in experimentation (or perhaps demonstration) remains uncertain for now, and we are committed to deepening our understanding of all its subcomponents going through new archival material. The final goal is to develop (and publish) a comprehensive biography of DM 1182. By piecing together its history and function, we aim to position it within the broader context of early 19th-century scientific research into electrostatics, using it as a case-study to show how scientific instruments can provide valuable perspectives on the development of knowledge and experimentation through the centuries.

References

[1] “Coulomb ‘sche Drehwage aus sechseckigem Sockel mit Glasschutzhaube Durchmesser 29 cm Gesamthöhe 46 cm – Coulomb ‘sche Drehwage nach Jelins Abänderung”, Deutsches Museum – Sammlungsmanagement, Grunddaten-Ausfruck, Obj. Id: 598591, Inv.-Nr. 1182.

[2] Charles Augustin Coulomb, 1784. “Recherches théoriques et expérimentales sur la force de torsion, et sur l’élasticité des fils de métal,” Mémoires de l’Academie Royale des Sciences de l’Institut de France, vol. 87: 229–269.

[3] Charles Augustin Coulomb, 1785. “Premier mémoire sur l’électricité et le magnétisme. Construction et usage d’une balance électrique fondée sur la propriété qu’ont les fils de métal d’avoir une force de réaction de torsion proportionnelle à l’angle de torsion. Détermination expérimentale de la loi suivant laquelle les éléments du même genre d’électricité se repoussent mutuellement,” Mémoires de l’Academie Royale des Sciences de l’Institut de France, vol. 88: 569-577.

[4] “LI.71. Eine Coloumb’sche Drehwage nach Yelin’s Abänderung um die abstoßenden Kräfte der Elektrizität durch Gewicht auszudrücken”, Ohm, Georg Simon 1850. Catalog der physikalisch-mathematischen Sammlung des Staates. Manuskript. Deutsches Museum Archiv (DMA) HS 1954-6.

[5] Heering, Peter 1992. “The Replication of the Torsion Balance Experiment: The Inverse Square Law, and Its Refutation by Early 19th-Century German Physicists,” in Restaging Coulomb: Usages, Controverses et Réplications Autour de La Balance de Torsion, ed. Christine Blondel and Matthias Dörries, Biblioteca Di Nuncius 15 (Firenze: L.S. Olschki, 1994), 47–66.

[6] Heering, Peter 1992. “On Coulomb’s Inverse Square Law,” American Journal of Physics 60, no. 11: 988–94, doi:10.1119/1.17002.