Why do old tube lights flicker? When tube lights become considerable old, they start flickering; i.e when the tube lights are switched on, they don't glow immediately, but start flickering. After some time they glow. Sometimes they become so old that they flicker continuously and have to be replaced with a new one. Why does this happen? Also, I have observed that a black mark appears at the end of the tube. What is this mark?
 A: Wikipedia mentions something similar to your situation:

The end of life failure mode for fluorescent lamps varies depending on how they are used and their control gear type. Often the light will turn pink (see Loss of mercury) with black burns on the ends of the lamp due to sputtering of emission mix (see below).

In thermionic emission, electrons pass from a cathode into a gas. Over time the "mix" that allows this to happen is sputtered away (i.e. ejected) by impacts from electrons and mercury ions. The black you see is the result of this mix.
Flickering is related to this. Eventually, the mix has trouble passing along enough electrons, resulting in flickering.
A: Its a chemical reaction caused by the heat and the florescent filament that causes the black marks. It only happens near the ends because that is where the energy is more concentrated. Why they flicker? well that can be for a few reasons. Corrosion to the metal connectors, or simply they no longer can hold the energy out put. Or simply they need to be re tightened.
