Discharge Through Gases

Gases normally insulate. Give them a high enough voltage at low enough pressure, though, and they glow and conduct, in patterns that led straight to the discovery of the electron.

Making a gas conduct#

A gas has few free charged particles. Cosmic rays create some ions, but they recombine about as fast. To pass a steady current we must keep producing charged particles: for example by applying a large voltage to a gas at low pressure, by heating a metal in a vacuum until it emits electrons, or by sending X-rays through the gas.

The discharge tube#

A sealed glass tube about 30 cm long and 4 cm wide has a metal electrode at each end and a side tube for pumping out the gas. The electrodes connect to the secondary of an induction coil: the one at the negative terminal is the cathode, the other the anode.

As the voltage is raised, sparks eventually jump. The least voltage that causes sparking is the sparking potential. Paschen found it depends only on the product of pressure and electrode separation:

What happens as the pressure falls#

Typical values for a 30 cm tube (the exact pressures depend on the tube, voltage and gas):

• ~10 cm of Hg: irregular streaks of light and a crackling noise.
• ~10 mm: the streaks broaden into a luminous positive column almost filling the tube, with a buzzing sound. Its colour depends on the gas: reddish for air, bright red for neon, bluish for CO₂.
• ~3–4 mm: the column shortens; a bluish cathode (negative) glow appears, separated from it by the Faraday dark space.
• ~1 mm: the glow detaches from the cathode, leaving the Crookes dark space next to it.
• ~0.1 mm: the column splits into bright and dark bands: striations.
• ~0.01 mm: striations and glow vanish, the Crookes dark space fills the tube, and the glass walls fluoresce (yellow-green for soda glass).
• Lower still: the current dwindles and the tube stops conducting.

Why the glow has this pattern#

• The few ions present are accelerated by the field; with enough voltage they ionize molecules they strike, and ionization snowballs. At low pressure, freed electrons travel far before attaching to anything.
• Positive ions accelerated into the cathode knock out more electrons. Light electrons race away while heavy positive ions linger, so a positive charge builds up near the cathode, making an intense field there.
• Electrons leaving the cathode travel about one mean free path before they collide and ionize, so the glow starts a little way from the cathode: that gap is the Crookes dark space.
• Passing through the positive charge in the glow region slows the electrons and they lose their ionizing power: the Faraday dark space (longer, because the field there is weaker). Reaccelerated, they ionize again, starting the positive column.
• Repeated cycles of ionizing, slowing and reaccelerating give the striations.
• As pressure drops the mean free path grows, so the Crookes dark space lengthens; when it exceeds the tube, electrons hit the glass directly and make it fluoresce.