Purpose: To demonstrate the characteristics of a gas discharge at various pressures.

*Demonstration not currently available in CHEMP 130.

If you need this demo sooner rather than later let me know and I'll move its priority up.

The apparatus consists of a vacuum system (forepump and oil diffusion pump), a discharge tube and a variac-controlled high voltage transformer -- 25 kV, SO BE CAREFUL.

Turn on vacuum system 1/2 hour before class so that no discharge can be seen when high voltage is applied (60 V on primary). Open leak and admit air (or neon) and observe:

• thin dark space next to electrodes (Aston dark space), due to inability of slow electrons from cathode to excite gas.
• thin cathode glow due to excitation of gas by cathode emitted electrons which have acquired more energy.
• cathode dark space where cathode-emitted electrons are now moving past and do not produce appreciable excitation.
• so-called negative glow extending a few centimeters due to secondary electrons from ionization of gas in the cathode dark space by cathode electrons.
• next is the Faraday dark space due to inability of the slow electrons from the negative glow to produce excitation.

Increase pressure by opening wide the lead and closing off the forepump valve. Structure of discharge contracts and changes character due to shortened mean free path of electrons and changes in potential distribution.

For more detail, see Encyclopedia of Physics, Vol. 22.

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