Electrical Discharges Gallery

Here are some photographs of various unusual or interesting glow discharges.  All

effects shown are easy to recreate, although various high voltage DC and RF power

supplies and a high-vacuum pump are required in most cases.  Pressures, where

given, were measured with a very accurate capacitance manometer.

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Nitrogen Afterglow in Air

One of the most interesting discharge phenomena in air, and one of the simplest to enjoy, is

the nitrogen "afterglow."  Shown here is a 2" diameter glass tube with no electrodes,

maintained at 0.36 torr.  In the upper image a discharge is being excited by radiofrequency

(RF) power from a vacuum-tube oscillator.  The oscillator is capacitively coupled to the tube

via a loop of wire at the very left.  A salmon-colored plasma fills most of the tube.  Switch off

the power and you see what is shown in the lower image--a greenish yellow luminescence

that persists and dies away exponentially over the course of about a minute.  This is the

afterglow.  Caused by the decay of excited states of atomic nitrogen, the glow shows up best

when no metal, hydrocarbons or other materials that catalyze the recombination of nitrogen

are present.  This means no electrodes are wanted in the tube.  50 watts of power at 30 MHz

should suffice to fully excite the tube, and even less appears to do the trick.  A diathermy

machine or amateur radio transmitter can source the power.  I use a Hartley oscillator.