Two 25 Watt bulbs in sockets, Variac, propane torch, hammer, baggie, wire cutters
Two 25 Watt bulbs in sockets

 

Code Number: 5D20.60

Demo Title: Conductivity of Glass

Condition: Excellent

Principle: Electrical conductivity

Area of Study: Electricity & Magnetism

Equipment: 

Two 25 Watt bulbs in sockets, Variac, propane torch, hammer, baggie, wire cutters.

Procedure:

Hook the two 25 W bulbs in series to the Variac and turn on so that the bulbs visibly glow.  Shut the Variac off and unscrew one of the bulbs.  Place the bulb in a baggie and gently break the envelope with a hammer.  Take care not to damage the inner glass stem.  Clip off the filament as close to the glass stem as possible.  Screw back into the socket.  Turn on the Variac.  The circuit should show no conduction (the undamaged light should remain off).  Light the propane torch and heat the glass stem.  When the stem starts to glow orange conduction should start to occur and be semi self sustaining (the undamaged 25 W bulb will start to glow).  When the glass stem cools conduction will stop.

References:

  • Chris Chiaverina, "A Simple Demonstration of the High-Temperature Electrical Conductivity of Glass", TPT, Vol. 52, # 1, Jan. 2014, p. 58.
  • Joe L. Ferguson, "A Simple Demonstration of Conduction in Glass at High Temperature", AJP, Vol. 58, #1, Jan. 1990, p. 90.
  • Richard J. Fitzgerald, "An Insulator with Conducting Electrons?", Physics Today, Sep. 2015, p. 18.
  • William Beaty, Tap-L conversations: "Microwave Oven Melts Beer Bottle", Nov. 29, 2003. 
Melt A Frickin' Beer Bottle!