PURPOSE: To show that glass is an insulator when cold, but when heated the resistance changes and it becomes a conductor.
DESCRIPTION: A light bulb is smashed with a mallet to obtain the glass stem. The filament is cut away to leave two disconnected metal wires in the glass stem. It is screwed into a lamp base that is connected in series to a 200 watt bulb. The base is switched on to connect it to 110 AC. A propane torch is used to heat the glass and cause it to melt. Once heated the glass will conduct and self-heat, glowing orange and causing the 200 watt bulb in series with it to also glow.
This demo can be a model for silicon semiconductors. Silicon itself alone is an insulator. But when it is doped with impurities such as germanium and boron the impurities can become conductive if a voltage is supplied. Similarly, glass is a silica and an insulator. But when doped with quartz, lead and other impurities and heated, the ions in it can then become free and conductive.
EQUIPMENT: The propane torch is stored in the general use cabinet.
Updated by Jun Qi in 3/9/2000