A Mysterious Coulomb Torsion Balance in the collections of the Deutsches Museum

At a recent (March 2023) workshop on “Material Culture in the History of Physics” held at the Deutsches Museum we were presented with a mystery object from the museum’s historical collection (DM 1182), which has previously been in the Bavarian Academy of Sciences (BAdW) since at least 1850, if not earlier.  Although the museum catalog describes it as a “Coulomb torsion balance with Yelin’s modifications,” the apparatus bears no resemblance to the very famous and oft-reproduced instrument by Charles-Augustin de Coulomb from 1784. It has no wires, no suspended items, and no scales for measuring. The instrument itself presents additional questions: its unusual red paint (only inside), imprecise construction, evident damage, apparent and missing pieces/parts of the apparatus, lack of documentation, and longevity in the collection all present analytical problems.  The aim of our contribution is to present the steps we have taken to analyze this artifact, and attempt to establish better provenance, identification, and contextualization. We will start with a description of the object, then try to understand the missing parts and the principle to arrive at hypotheses of functioning. We will draw on several sources, including archival documents from the BAdW, historical catalogs by Ludwig Boltzmann and Georg Simon Ohm, comparison of this instrument with contemporary electrical devices and with other instruments from the BAdW collection), and with the writings of Julius von Yelin, to whom the instrument has been attributed since 1850.