Dark moving spots on flourescent tubes

[Gav1n]

I have noticed that some of the flourescent tubes used to light train carriages on the District Line have dark spots moving down them. The speed and distance between the spots varies between lights, but the spots are always regularly spaced. Has anyone else ever seen this phenomenon, or know what causes it?

[Hippy]

The flourescent tubes light up because a gas inside the tube forms a glowing plasma caused by a high voltage across the ends.

 

In the Underground trains the voltage is created by an oscillator and sometimes the frequency of this oscillator causes a standing or slowly moving wave in the plasma. The frequency is higher than the normal domestic lights that use the 50 cycles per second mains AC to excite a choke and produce the high voltage spikes to fire the gas into a plasma.

 

[ideasman]

i had the same problem with the tubes in my kitchen but found that rolled up copy of the weekly news usually did the trick!