Improved glow lamp - air

The pictures above shows the improved glow lamp operating in low-pressure air at a few millibars (left) and below the millibar level (right). Because of the AC operation, now both electrodes light up. As with of the "standard" air glow lamp (see there) a significant UV-A emission arises from the discharge, which results in a strong blue fluorescence of the underlying piece of paper. Interestingly, at very low pressure ions are energetic enough to cause the release of sodium upon impact with the glass capillaries of the electrodes (see the orange emission in the right image). From a chemical point of view this is of course undesirable as the released sodium is bound to react with the gas fill, especially oxygen. So, a stable lamp operation requires a fill pressure that is not too low (left image).