Late 19th-century Scientific Instrument
This late 19th-century scientific instrument — from the “Physical Cabinet”at St. Ignatius College — was designed to show that charge accumulates with the greatest density in areas where the radius of the conductor’s curvature is smallest: small spheres hold lots of charge relative to their surface area, and large spheres hold very little charge per area. (Yes, that’s counter-intuitive!) One can adjust all of the spheres such that they are touching, connect them to a static generator and raise them to the same voltage, and then separate them and measure the charge density and see that the smallest sphere contains the most charge per area. (There were little wands made for doing this — ours are missing.)
This means that the electric field is greatest at the pointy ends of irregular conductors. This is part of the reason why your fingertips spark when you touch a doorknob in winter — big news in the late 18th and early 19th centuries.
The exhibition is grateful to scholars who helped solve the mysterious identity of this instrument: Juliet Burba and Adrian Fischer at the Bakken Museum, Minneapolis; and Jamie Day at the The Monroe Moosnick Medical and Science Museum (Transylvania University, Kentucky).