The Luminous Pane.
Fig. 37.
This easily made exhibit (Fig. 37) is one that is susceptible of quite a number of applications. In its simple form it is but an enlarged version of the rubber sheet scattered with carbon dust. The old way to make it was to take a plate of glass and cement on one face of it a sheet of tinfoil, using shellac varnish preferably. When dry, the tinfoil was scored across and across in such manner as to divide it up into little squares or diamonds. When the current was applied to each end of the plate, the spark divided into innumerable little ones; between each bit of tinfoil and its neighbors there would be many little sparks, and the effect was very pretty, somewhat as was described before when the carbon dust was strewn between the electrodes. It is more easily and quickly prepared by giving a sheet of glass a coating of shellac varnish, and then sparingly dusting any powdered conductor over its surface, using perhaps carbon dust or filings of metal. By cutting out a stencil from a piece of thin card and laying it over the sparkling plate, the design shows out very strikingly, and various designs in stencils can be prepared, different powdered conductors giving different colored sparks.
A long glass tube moistened inside with mucilage or shellac varnish and then having some conducting dust shaken through will also give quite a pleasing effect.