We usually think of glass as a good electrical insulator; this, however, is not always the case. There are several ways to show that glass becomes conducting at high temperatures,1,2 but the following approach, devised by Brown University demonstration manager Gerald Zani, may be one of the simplest to perform.

Topics
Electric lamps, Electrical insulator, Electrical conductivity, Temperature coefficient, Glassforming technique, Electrical resistivity, Negative resistance, Chemical bonding, Educational aids
1.
An early version of this demonstration appears in Richard Sutton's 1938 Demonstration Experiments in Physics.
2.
I was first made aware of the high-temperature conductivity of glass through a presentation by physics teacher Bob Froehlich at a meeting of Physics Northwest, a Chicago-area AAPT affiliate.
© 2013 American Association of Physics Teachers.
2013
American Association of Physics Teachers
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