The breakdown voltage in air at STP is about 3 MV/m. Pressure in the chamber is reduced as a voltage (V = 2150V) is applied resulting in the air becoming ionized. When an electrical arc (spark) is generated between the two conducting spheres the measured voltage drops to V = 1270V.
As the pressure continues to drop the mean free path of the electrons and ions increases, and the arc becomes a plasma tube. This is referred to as glow discharge. The voltage stabilizes at V = 311V.
Voltage measurements (Vmax) via 200:1 voltage divider. f = 7 kHz.
Vrms = 0.707*Vmax Vmax = Vrms/0.707
Viewing the electrical arc and glow discharge is best in a darkened room. It is easily projected on a screen with a camera. The line spectra of nitrogen are easily viewed in a dark room with a diffraction grating.
This is historical scientific equipment, manufactured by Max Kohl A. G., Chemnitz, Germany. This company was established March 4, 1876 . The date this device was purchased is unknown. One of their catalogues can be viewed online from The Smithsonian Institute . Select pages from the catalogue are attached.
Electrical Arc (Spark)
Glow Discharge Close Up
Increase the voltage as the bell jar is being evacuated. The full range of the power supply knob is not available; listen for the sound generated (7 kHz) by the power supply as you rotate the knob clockwise. Once glow discharge is achieved, you can reduce the power without affecting the glow (negative resistance). If the power suddenly stops, simply turn the knob counter-clockwise until the power is on again (you’ll hear the sound).
Nitrogen from the atmosphere is at low pressure and high voltage. The line spectra of N can be observed with a diffraction grating.