ihutc: This is a replica Geissler tube. I don’t know what it does, but here’s an excerpt from the label at the Science Museum where I took the photos. The two tubes here create spectacular effects of electricity in a near vacuum. Geissler not only managed to evacuate and seal off the tubes without contamination, but made use of the coloured effects of different gases, flourescent liquids and uranium glass. Unfortunately the originals are slightly radioactive because of the uranium glass so cannot be displayed.