Q: I light my halls wth neon-bulb 鈥淕lowlights鈥� which glow a steady orange during the day. Late at night, however, they flicker dramatically, sometimes going out completely for a full second or more, then blinking on again. If I shine a small torch at them, they revert to their steady daytime glow. Putting two lights in adjacent sockets so that one is lit by the other does not stop the flickering. What is going on?
A: A neon glow lamp glows when the voltage across the electrodes exceeds the breakdown voltage of the rarefied gas inside. The current flow ionises the gas. Ions then recombine to create light, the colour depending on the kind of gas. At the end of the half-cycle of alternating current mains voltage (the mains supply switches its polarity 100 times per second) the arc of light goes out, and strikes again during the next half-cycle.
The breakdown voltage depends on a number of things: electrode spacing, electrode material, sharp edges on the electrodes which cause higher voltage gradients, gas pressure and anything which excites the atoms at the surface of the electrodes. As the lamp is used, material from the electrodes is lost (sputtered) by bombarding ions, and ends up on the inside surface of the lamp, where it traps gas, thereby reducing the gas pressure. This increases the breakdown voltage (also darkening the glass) and the arc strikes later in the cycle and eventually not at all.
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If the mains voltage varies because of power demand, the peak voltage may not be enough to ionise the gas, or the current may only flow one way through the gas, causing erratic behaviour. Shining a light onto the electrodes excites the surface atoms enough to allow them to release electrons at a lower voltage. The shorter the wavelength of light, the better the effect.
In the past, another (environmentally unfriendly) way to reduce the breakdown voltage was to add some radioactive material to provide a plentiful supply of free ions. The questioner could try holding an old watch with a radium dial near his neon lamp to observe this.
A: The reason having two lamps doesn鈥檛 help is that both lamps turn off twice during each AC cycle. Thus they are both off at the same time and can鈥檛 possibly help each other.
and in any case 鈥�
A: The light of an adjacent neon bulb, being orange, does not contain wavelengths short enough to liberate the extra electrons from the nickel electrode surface, however bright the bulb might be.
A: Many years ago, I had a similar problem with a neon gas-filled relay, used as a part of a cosmic ray telescope. The device worked perfectly in the laboratory on the Earth鈥檚 surface, both in light and dark. However, its operation became very uncertain when used in the dark in the depths of the Underground station at Holborn (a good few metres below the surface) although it still operated well in the light.
Generally, the relay either failed to operate at all or did so at some indeterminate time after receipt of the triggering pulse, and then very feebly. We concluded that it needed a little energy from an external source to provide enough ions to initiate reliable triggering and these were normally obtained from either the photons of the incident lighting or from background ionising radiation, probably cosmic rays.
In the Underground, the background cosmic radiation was very much weaker than on the surface, and insufficient for correct operation in the dark. Certainly, bringing a weak radioactive source close to the relay cured the problem, as did a very low level of lighting. I taped a torch bulb to the relay and it then performed perfectly 鈥� which was more than the rest of the equipment did.