5D40.50 - Neon Bulb Trigger - Avalanche Trigger

Code Number:
5D40.50
Demo Title:
Neon Bulb Trigger - Avalanche Trigger
Condition:
Excellent
Principle:
Switch Trigger
Area of Study:
Circuits

Disclaimer

Equipment:
0 - 150 Volt Power Supply, Neon Bulbs (Wheat and Bulb Style), and Red, Green or UV Laser Pointer, Camera Flash Unit.
Procedure:

Two neon bulbs are available for this demonstration.  We usually use the large bulb for maximum classroom visibility.

Plug the desired neon bulb into the power supply and turn the voltage up until the bulb comes on and note that voltage.  Turn the voltage down until the bulb is completely off.  Turn the voltage back up until it is about 5 to 10 volts below the voltage that will turn the bulb on.  You can now trigger the bulb to turn on by shining a laser onto the filament on the inside of the bulb.  The red laser will have no effect.  The green laser will trigger the bulb at about 5 volts below the turn on voltage of the bulb.  The purple 405 nm laser will trigger the bulb from 10 to 15 volts below the turn on voltage.  The flash unit will also trigger the bulb at about the same voltage as the purple laser.  

For the small "wheat" style neon bulbs the voltage needed to trigger it with the green laser pointer will be about 62 volts.  For the UV laser pointer the voltage needed to trigger this bulb will be about 55 volts.  Note that once this bulb is triggered you will need to turn the voltage down to about 50 volts to turn off the bulb.  An interesting additional effect can be observed with this bulb.  Find the minimum voltage that will turn on the bulb using the purple laser.  Then turn down the voltage 1 or 2 more volts.  Now when you shine the purple laser onto the filament, the bulb will turn on, but when you move the laser beam off the filament, the bulb will go back out.  Very Cool!!

 

References:
  • H. Richard Crane, "How Things Work: Touch-Panels in Elevators, and Idiosyncrasies of Gas Tubes", TPT, Vol. 21, #6, Sept. 1983, p. 402, also A Potpourri of Physics Teaching Ideas - Electricity and Magnetism, p. 157.
  • Jesus Gonzalez-Laprea, L. J. Borrero-Gonzalez, "Photoelectric Effect Measurements on a Conventional Neon Bulb", AJP, Vol. 89, #10, Oct. 2021, p. 969.
  • Cliff Bettis and Sam Sampere, Tap-L conversations, Feb. 20, 2020.

Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.