Conductivity of Glass

Insulating glass becomes a conductor of electricity when heated red-hot with a blowtorch. (m) (T+)

What it Shows

At room temerature, glass is almost as good an insulator as hard rubber. When heated to 1000 K, however, glass has a resistivity of less than 107 ohm-meters (Purcell1 fig. 4.8 pp 140). As glass becomes molten the once immobile ions are able to drift further between collisions under the influence of an applied electric field (Purcell pp 139). We can dramatically observe this decreased resistance using a blowtorch and a few incandescent lightbulbs.

Conductivity of Glass

How It Works

Two ceramic lightbulb sockets are wired in series to a household AC power cord. When two bulbs of the same Wattage rating are screwed into the sockets and the cord is plugged in, they both pass the same amount of current and so they both light with the same intensity. When one bulb is unscrewed, the circuit is broken and the other bulb goes out. If we can replace the missing bulb with a conductive material, the circuit will once more be complete and the remaining bulb will light again.

Conductivity of Glass

Setting It Up

Unplug the cord and install a prepared, broken bulb into the empty socket. The bulb needs to be broken carefully to preserve the glass wire mount in the base, and the contact and support wires should be trimmed, leaving a glass stem in the screw-in base. Safety glasses and leather gloves should be used throughout this procedure.

Plug in the cord. The intact bulb remains unlit because the resistance of the glass in the other socket is too high at room temperature to pass enough current. With a MAPP gas torch, heat the glass stem that protrudes above the socket. Take care to control the flame direction to avoid heating any other part of the demo!

As the glass with the live contact wires glows and begins to melt, it becomes a conductor and is able to carry enough current to light the other bulb. Once the glass slumps a bit it should be hot enough to conduct pretty well.

Remove the flame once the bulb is lit. As the glass cools, the conductivity decreases again and the once bright bulb starts to grow dimmer. Blowing air at the hot glass stem cools it enough to turn off the light!


With a 75 W bulb, the demo can reach a steady state, where there is enough self heating in the glass stem to keep the bulb lit at a low level. Thanks to Gerald Zani at Brown ( for showing us how to do this demo with incandescent bulbs!

1 Edward Purcell, Electricity and Magnetism - Berkeley Physics Course Vol 2, 2nd edition (McGraw-Hill, 1985)