Diagrams, 'Formation of spark tracks' and 'Path of incandescent particles at the poles' by John Joly Two small pencil diagrams, labelled figures 10-11, illustrating Joly's observations on the spark discharge. The diagrams appear as figures 3 and 4 respectively in the published paper. Includes notes to the engraver. Subject: Electricity Prints of these diagrams were published in volume 47 of the Proceedings of the Royal Society alongside the paper 'Observations on the spark discharge'. 1889