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.
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.
I'll throw this into the mix. If I plug in one end of my guitar cable to my amp, turn the amp on, bring up the volume a little, and touch only the center contact on the other end of the cable, I get a loud 60 Hertz buzz from my amp. I believe this is because my body is acting as an antenna - picking up some radiation from the house wiring, which is typically unshielded.
Could this "antenna effect" also play a part in triggering the fluorescent lamps to light when they are touched?
I also know that if you take a fluorescent tube and go stand under a high voltage line, it will glow because of the very high E-field levels.