#7a.   The Fluorescent Lamp:
      A plasma you can use

    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?


    Good question. If you have studied electricity, you surely learned there about Ohm's Law, by which the current flowing through a device is inversely proportional to its electrical resistance R. Double the resistance R and only 1/2 of the current gets through, replace it with one 10 times larger and only 1/10 as much manages to flow. It is a bit like water flowing in a pipe--if you make the pipe 10 times narrower, then (other things being equal) only 1/10 as much water flows through.

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.

  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.

(And if you think Ohm's law is badly violated by fluorescent lamp plasmas--just wait till you read about the ring current, the electric current carried around Earth by trapped ions and electrons of the radiation belt. That current needs no voltage at all, it circulates just because of the trapping of the plasma!)

  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.