(Files in red–history)
4a. Electric Fluid
5. Field Lines
5H. Faraday 1846
5a-1. EM Induction--1
5a-2. EM Induction--2
6. EM Waves
7. Fluoresc. Lamp
7H. Langmuir, 1927
8. Positive Ions
8H. Arrhenius, 1884
9. Magnetic trapping
You may have noticed in the drawing (reproduced here) that the circuit of the fluorescent light fixture included a "ballast coil.," You might also have noticed such coils in fixtures in your home, often encased in a rectangular box. Ordinary hot-filament lightbulbs are connected directly to power lines, but fluorescent lamps always receive their current through a ballast. Why?
A greedy conductor defies Ohm's law!
Well, in case you thought that Ohm's law was a universal law of electricity--think again, because it isn't. Metal wires satisfy it fairly well, although their resistivity varies with temperature: a cold lightbulb filament has only 1/5 the resistance of a hot one, so that initially the lamp draws a 5-fold current, which helps switch it on quickly. But plasmas do not satisfy it at all. The resistance of your fluorescent lamp is not fixed, it depends on the current carried: the greater the current, the smaller the resistance.
Put in other words, the plasma is a greedy conductor of electricity. Suppose it has just enough free electrons to get a current started. The current causes ions and electrons to move rapidly and to collide violently, and those collisions strip additional electrons off atoms of the gas. Additional electrons increase the current, causing more collisions and producing still more electrons, which create more current, more and still more... In this way, if a fluorescent lamp were directly connected to the power lines, unprotected, its current would rapidly grow until something gave way. The tube might heat up and explode, the wiring might melt... or more likely, the fuse or circuit breaker which protect the fixture would stop the current.|
Why then a coil and not a resistor? Because the tube is fed by an alternating voltage, which rises and falls 120 times a second (in the USA; 100 times in Europe). Its electrical current sloshes back and forth, 60 times a second in one direction, 60 times in the opposite one. In between, 120 times each second, the voltage drops to zero and the tube is extinguished, since plasmas react very quickly. Somehow, it must be relit!
A ballast coil can do that. In an alternating current, it acts a bit like a resistance. As the current rises, it absorbs energy from it to build up its magnetic field, slowing down its growth. Then, when the voltage drops to zero, the stored magnetic energy produces a voltage surge which relights the tube. You will not usually see the fast flickering of the light, except maybe if you illuminate a rotating fan, when (at the right speed) its motion seems to stop. (Note: compact fluorescent lamps now exist in which the ballast coil is replaced by a more complex electronic circuit. The flow of electric current is then limited by transistors.)
And what about this "fluorescent" thing? The mercury atoms in the plasma generate light very efficiently, but much of it is ultra-violet (UV), invisible to the eye and harmful to it (or rather, it would be, were it not absorbed by the glass). The solution is to coat the inside of the tube with a glow-in-the-dark (fluorescent) paint, which absorbs the UV and re-emits its energy as visible light.
All other plasma lamps--sodium and mercury streetlights, neon lights etc.--require ballast coils, too. Recently, small fluorescent lamps have appeared on the market, which screw into the socket of a regular lighbulb. They have transistor circuits replacing the coil, and although they cost more than filament lamps, they are (like other fluorescent lamps) much more eficient.
A few words about safety
A low-intensity hum is probably no cause for alarm, although it can be annoying. But if the hum gets really loud, it my be safer to replace the coil or the fixture. Electric transformers are also constructed around stacked iron plates and are subject to the same problem.
Questions from Users:
*** Fluorescent lightbulbs
*** Fluorescent tubes lighting up near high voltage
Next Stop: #7H. Plasma--History