On Thu, 25 Nov 1999 18:48:50 -0500 Wagner Lipnharski writes: Any light beam over > the > lamp changes the jumping ionization to a steady one. The process that occurs inside a neon lamp is much more complicated than it may first seem. I'm not an expert in physics but since the project I'm working on at work is very similar to this I've been forced to learn a little. The description below is by no means complete, but it gives an idea of the major effects involved. Whole books have been written about the subject. Just placing neon gas in an electric field will not make it glow. It has to become ionized by something hitting the neon atoms with enough energy to remove an electron from them. In a neon lamp this would most likely be an electron. If and when an electron leaves the cathode of the lamp it accelerates toward the anode, likely striking one or more neon atoms along the way. If these neon atoms become ionized they now have a postive charge, thus they start moving toward the cathode. At or near the cathode, there are enough slow-moving electrons to de-ionize the neon. When the neon atoms regain their electron they glow. The glow in an ordinary lamp appears near the cathode. The neon atoms and ions strike the cathode with enough force to dislodge more electrons. Thus, once the process has started, it will continue in a chain reaction manner. To keep from burning the lamp out with too much current a resistor is placed in series, this allows the voltage to drop to the point that the glow stays under control. As you might expect, using a larger resistor leads to a dimmer glow but longer life. The resistance doesn't affect starting because the lamp current is zero until it starts. So far the lamp was considered in DC operation. In an AC circuit the lamp has to re-start for each power line half cycle. An old lamp that doesn't start well would not start on every half cycle thuse it would flicker rather than glow steadily. I think the most plausible explanation for Wagner's results is a photoelectric effect at the cathode. With light striking the cathode the lamp is able to start on every half cycle. (Visible light does not have enough energy to ionize the gas directly). Nuclear radiation would ionize the gas but I don't think it is used in little neon glow lamps. There is always of course our friendly "background" radiation. Fluorescent starters often have nuclear materials in them because they need a carefully controlled starting voltage. The same effects occur with about any gas in the lamp, however neon has been selected because it has a strong glow at a clearly visible color. Also, the noble gases (helium, neon, argon, krypton, and xenon) have an effect called the "metastable neutral" that allows a gas atom to become excited, but not ionized. These metastable neutrals help to remove electrons from the cathode, allowing the lamp to operate at a lower voltage than would be necessary with other gases. Applying a magnetic field perpendicular to the path between the cathode and anode causes the electrons to move in a curved path rather than straight between the two. With a lower pressure of gas in the lamp, operation is improved because the electrons are more likely to encounter atoms of gas. If the electrodes are set up properly it is possible to make the electrons move essentially in circles, collecting any gas atoms at all that may be present. > I didn't measure the AC current at the neon lamp, but it could > change in > presence or not of light... a photo sensor? Yes it does but it could have an "avalanche" effect. Without any neon at all it would be a phototube, detecting light by producing a tiny current of electrons only. Filled with gas and left in the dark, nuclear radiation will start it. This is the effect found in a Geiger tube. Of course, devices specifically designed to detect light or nuclear radiation are designed with different materials and structures to optimize these effects, but the same physiscs can occur in a neon lamp. ___________________________________________________________________ Get the Internet just the way you want it. Free software, free e-mail, and free Internet access for a month! Try Juno Web: http://dl.www.juno.com/dynoget/tagj.