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

What that Shows

At room temerature, glass is nearly as great an insulator as difficult rubber. When heated to 1000 K, however, glass has actually a resistivity of much less than 107 ohm-meters (Purcell1 fig. 4.8 pp 140). Together glass i do not care molten the as soon as immobile ions space able come drift further between collisions under the influence of an applied electric field (Purcell pp 139). Us can significantly observe this reduced resistance using a blowtorch and a few incandescent lightbulbs.

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How that Works

Two ceramic lightbulb sockets room wired in collection to a household AC strength cord. When two bulbs the the same Wattage rating room screwed into the sockets and also the cord is plugged in, lock both pass the same amount that current and also so lock both light v the very same intensity. As soon as one bulb is unscrewed, the circuit is broken and also the other bulb walk out. If we have the right to replace the lacking bulb through a conductive material, the circuit will once much more be complete and the continuing to be bulb will certainly light again.


Setting the Up

Unplug the cord and install a prepared, damaged bulb into the north socket. The bulb requirements to be broken carefully to maintain the glass cable mount in the base, and also the contact and also support wires must be trimmed, leaving a glass stem in the screw-in base. Safety glasses and leather gloves must be provided throughout this procedure.

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Plug in the cord. The intact bulb remains unlit due to the fact that the resistance of the glass in the other socket is also high at room temperature come pass sufficient current. With a MAPP gas torch, warmth the glass stem the protrudes above the socket. Take care to regulate the flame direction to prevent heating any other part of the demo!

As the glass with the live call wires glows and also begins come melt, it i do not care a conductor and is maybe to bring enough current to irradiate the other bulb. Once the glass slumps a bit it have to be hot sufficient to command pretty well.

Remove the flame as soon as the pear is lit. As the glass cools, the conductivity reduce again and also the as soon as bright pear starts to flourish dimmer. Blowing air at the hot glass stem cools it sufficient to revolve off the light!


With a 75 W bulb, the demo deserve to reach a steady state, whereby there is enough self heater in the glass stem to store the bulb lit in ~ a short level. Thanks to Gerald Zani at Brown ( for mirroring us how to execute this demo v incandescent bulbs!

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