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Tesla and the photoelectric effect

  1. May 29, 2005 #1
    So someone pointed me to a patent of Tesla's (or two, rather, but basically the same thing):

    * US Patent 685, 957: Apparatus for the utilization of radiant energy

    * US Patent 685, 958: Method of utilizing radiant energy

    (You need a TIFF viewer and then click on the "images" link.)

    They said that this is a rudimentary solar cell. I said "no way!", but then I looked at it more in depth, and it does seem to be an implementation of the photoelectric effect for collecting solar energy.

    UV or X-rays of higher energy (frequency) than the work function of the metal (energy required to knock an electron from a metal's surface to infinity) knock electrons off the metal and give it a positive charge. Connecting it to a capacitor and then to the ground allows you to charge the metal plate up a lot more than before, because the capacitor "condenses" charge into a smaller area/volume (hence the older word "condenser" which is still used in every language other than English. does anyone know who invented the term "capacitor?")

    So I built a little thing to test it. I took a coat hanger and sanded off the varnish, and soldered one piece to the negative side of a 4700 uF electrolytic capacitor. I soldered the other piece of coat hanger to the other side and made a triangle shape. Then I scrunched up an aluminum pie plate type thing (like thick foil) on the triangle thing to make the "collecting" plate.

    I stuck the straight piece maybe three inches into the ground so the big plate is on top, horizontal, and shading the rest. Anyway, after leaving it like that for a few minutes in bright sunlight I measured -27 mV. :-(

    Just waving the probes around in the air makes it display 200 mV or so. Plus, the voltage on the plate is supposed to be positive because it has had electrons removed from it. So my experiment was a failure.

    So there are some possibilities that I see for why mine didn't work:

    * Photoelectric effect doesn't work in air (because the electrons bounce off air molecules until they have slowed down enough to fall back onto the plate?)

    * Didn't leave it there long enough. (I felt kind of silly measuring a lasagna lid. Gotta find a place with no one around.)

    * Capacitor thing with one end stuck in the ground doesn't really work that way. (I brought this up in another thread.)

    * Because the aluminum is covered in oxide (although oxide coatings reduce the work function for vacuum tube filaments, maybe they prevent photoelectric effect from working at all?)

    * I know pointiness and crinkles change the behavior of a charged metal object. It is easier for them to give off electrons at sharp points? Not sure if this has anything to do with it.

    It seems like it should work, though, since a similar experiment like this works: See Figure 3 or here.


    * It's in a vacuum

    * There is another plate to catch the electrons that have been kicked out

    * It operates in a more standard circuit-type way, whereas Tesla's is more a "unidirectional", "broken-circuit" charged object type of thing.
  2. jcsd
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