File:Fluorescent wear - mercury adsorption.png

English: A pair of fluorescent tubes that have been constantly lit (never turned on or off). The mercury vapor in the tubes slowly adsorbed into the glass over a long period of time, reducing the pressure in the lamps, until only the argon fill maintains the current flow. No longer able to operate as an arc lamp, the tubes become glow discharges, dimming considerably and eventually displaying "Faraday cones", which light up the fluorescent coating to appear as rings. To the naked eye, the rings appear to move from the ends toward the center, but at 1/4000 of a second, the camera depicts them in their true form, moving toward the cathode. During this particular half-cycle, the upper tube's cathode is on the left while the lower tube's cathode is on the right. The cathodes alternate to the opposite sides of the tubes for the following half of the cycle.