By Oli Usher, on 9 June 2014
When metal objects are given an electric charge, the charge is not evenly spread through it. It collects on the surface, and in particular on parts of the surface which are curved.
This object from the collections of the Department of Physics & Astronomy is a historic teaching device that would have been used to demonstrate this phenomenon in times past.
Sitting atop an insulating stand, the device would be charged up and retain the charge long enough for a lecturer to carry out a demonstration. Using a small square of metal on an insulating handle, attached to an electroscope, the demonstrator would be able to show that the charge was concentrated on the sharply curved area on the right hand side in this photo.
Photo credit: O. Usher (UCL MAPS). Acknowledgement: Jim Grozier