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In my old house there are two old tube lights. Some times they don't start properly, (specially at evening time, may be it is because of low voltage), they starts flickering i.e. on and off continuously. And when my elder brother touches them, mostly at center they just goes on. Even i have tried many times, the touching works, and even after removing the hand, once it is on(working properly), doesn't stop them, and they remains on. So why and how touching them works. Sorry for my bad English. Thanks in advance.

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up vote 7 down vote accepted

I've found this to be the case, too. Generally, my shop lights will flicker when turning on, especially when it is colder outside .

There are two basic phases to this kind of light bulb: a start-up phase, and an operating phase. The start-up requires more voltage, because you are initiating the plasma stream between the terminals of the bulb.

So, these bulbs are filled with gas, generally mercury vapor. When you turn it on, you're really creating an electrical arc at the end of the bulb. This arc ionizes the gas, and that is the flickering you see. Eventually, the entire length of the bulb can arc, and a plasma stream is created (plasma is just a gas of free-moving ions and electrons).

When it is cold, or if the starter arcs are wearing out (and can't produce sufficient voltage), then the flickering lasts longer. By touching the bulb, you actually provide a shorter path for the plasma stream! In effect, you get two short plasma streams, each terminating at your touch point. Then, once the plasma streams interact, and since the voltage required to maintain the stream is less than the voltage needed to establish it, the streams merge into one and the lamp will light.

This is similar to some "glowing orbs" you see in gift shops or museums... a glowing plasma (looks like lightning inside a clear spherical shell) will arc and bend toward your touch, since you are providing your own set of electric and magnetic fields.

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Sorry @OrangeWombat I did not want to downvote, my finger hit the wrong arrow :(. I will try to make a small edit and then I think I can change to what I wanted to vote. –  anna v Oct 23 '12 at 16:22
@annav, no worries! :) –  OrangeWombat Oct 23 '12 at 16:49
thanks for your answer. But i'm thinking, isn't it then dangerous to touch the tube lights, what amount of voltage and current the plasma has, and why can't we feel the current flowing in our body, because even we can the feel current while touching cable or telephone wires. Please can you explain these things –  android developer Oct 26 '12 at 6:31
It is not dangerous to touch the light, because you're not actually acting as a ground. I'm sorry my explanation was a bit vague on this point. Your touch generates a capacitance field, and the change in capacitance is what affects the plasma. This is the same effect that is used in "touch switches" on metallic fixtures, like some lamps. See this Wikipedia article for more detailed information about capacitave switches. –  OrangeWombat Oct 29 '12 at 0:00

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