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Are the ions in a fluorescent lamps concentrated near the electrodes , and just the electrons move to the other electrode or the positive mercury ions move too?

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The speed of the ions is slower than that of the electrons by a factor of the ratio of masses (to first order) which is of order 40,000, and in a alternating field they don't go far enough to bother ourselves about before the field reverses.

So, in principle yes, they drift, but they don't move far enough to accumulate near the cathode and for practical purposes you can ignore it.

More over, if you did get a large separation it would surpress the field.

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So at any given moment , will there be positive ions in the tube ? if yes, is this number constant, depending on the volume , voltage or whatever? –  Abdelrahman Esmat Jan 19 '13 at 16:46
    
"Fluorescent lamps may get dim at one end with DC. Since the mercury vapor ionizes more easily than the argon, some of it exists as positive ions. This can cause the mercury to be pulled to the negative end of the tube, resulting in a mercury shortage at the positive end. This is more of a problem with longer length and smaller diameter tubes. ... Some ... have special switches to reverse polarity every time the fixture is started. This balances electrode wear and reduces mercury distribution problems." repairfaq.org/sam/dlamp.htm –  endolith Dec 2 at 15:48

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