Bringing out the wheel-shaped Wimshurst machine once again, today’s electrostatic demonstration involves a spark discharge into a slab of resin. The resin is an insulator, which means that charge does not easily move through it. An excess of charge carriers (which we know today as electrons) cannot quickly disperse; they are trapped in the resin, and a lingering electric field is present along the path of the spark. Similarly, if the spark discharge creates a dearth of charge carriers, electrons elsewhere in the resin cannot rush in to fill the resulting void. Once again, a lingering electric field is present.
Last month, I was quoting the 18th-century electrical scientist Georg Christoph Lichtenberg. It was he who first employed fine powders to make visible the track left by a spark in an insulating material. Particles of the powder, attracted by the electric field, accumulate on the surface of the resin. The intricate and beautiful patterns, left in the wake of the discharge, become visible. They are known as Lichtenberg figures.
The video here shows that discharges from the positive terminal leave patterns with a different appearance than discharges from the negative terminal.
For more about Lichtenberg figures, I recommend this account by Bert Hickman, a passionate lover of high-voltage phenomena and a skillful engineer. Bert is a part of a community of artists who create beautiful works with Lichtenberg figures at their heart.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.