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Tesla patented a device for gathering energy from light, using the photoelectric effect. (US 685,957 - Apparatus for the Utilization of Radiant Energy):

Telsa's photoelectric effect generator

Basically just a sheet of "highly polished or amalgamated" metal (top) connected to a capacitor (the ⊔⊓ shapes) connected to the Earth. (The other stuff on the right of the image is a load intermittently driven when the charge on the capacitor rises high enough.)

The patent says it would be able to collect energy from the sun. Would this actually work? Why or why not? Does it need to be in a vacuum? The patent seems to say that it works better in a vacuum but that it is not required. Would the plate stay charged after electrons are thrown off, or would they be attracted and drift back to it and neutralize it? How does the capacitor to earth make this better than a plate hanging in space? It allows more charge to be stored for the same work function voltage of the metal?

The patent demonstrates that he doesn't understand the cause of the photoelectric effect ("sources of such radiant energy throw off with great velocity minute particles of matter which are strongly electrified, and therefore capable of charging an electrical conductor"), but he understands the results.

(Incidentally, I have tried building this with a piece of aluminum foil, electrolytic cap, and wire strong enough to stick a few inches into the ground. I didn't see any voltage with a multimeter in bright sunlight.)

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Extra image is not necessary, but you may want to add the description of the middle device. This image is very unclear. –  hwlau Dec 8 '10 at 9:26
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Exegese of that "middle" device is rather easy, it is an electromagnet coupled to a cam wheel by a ratchet drive. This will make revolutions going on in the same direction although the armature of the solenoid makes a reciprocating movement. The thing more to the right side is a electrostatic switch I suppose. The idea is that those two leaflets hanging down like in a electrometer will contact after some potential is reached. –  Georg Jan 31 '11 at 16:40
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Electrolytic capacitors have low parasitic resistance (leakage current) and a high capacitance. These two features combine to prevent you from seeing any voltage (it will leak off too quickly). Follow the advice of Tesla and go with a high quality, high voltage capacitor. I suggest teflon. Make sure that your voltage measuring device doesn't short out the capacitor. To test this, charge the capacitor with a voltage source (be careful to not kill yourself, use maybe 12V), and see if you can detect the charge. –  Carl Brannen Jul 13 '11 at 11:18
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What Tesla was seeing was indeed the photoelectric effect. This is why he thinks that the beam consists of "positively charged particles". So it helps to use a metal with a low work function. This is a problem as the good metals are too reactive. But the shortest wavelength hitting earth is 100nm or around 12.4 eV and this is enough to knock electrons off of any metal. –  Carl Brannen Jul 13 '11 at 11:27
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@Georg: Yes. Did you read my comments? –  Carl Brannen Jul 14 '11 at 18:06

2 Answers 2

up vote 10 down vote accepted

The energy needed to remove an electron from a solid is called the work function. For most metals you would need UV photons (300 nm for Aluminium) that rarely reach the Earth's surface. Visible light can eject electrons from alkali metals, but the quantum yield (the probability of electron emission per incident photon) for pure metals is low (probably less than 1%). Materials like CsTe that are used in photocathodes have efficiency up to 40% (at certain wavelength) but they are expensive and difficult to handle in open air. Silicon solar cells also utilize photoelectric effect and compared to metals they are efficient and inexpensive.

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I would say that it is an issue of efficiency. In principle it could work, but there are more efficient ways. –  Vagelford Dec 8 '10 at 9:33
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Not exactly. For visible light the efficiency of most metals will be exactly zero because the photon energy is just not high enough. –  gigacyan Dec 8 '10 at 9:46
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So it is a issue of efficiency. –  Vagelford Dec 8 '10 at 10:47
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An efficiency of exactly zero is not really "an issue of efficiency". –  endolith Dec 8 '10 at 21:05
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If it was exactly zero, you would be right. But as you say in your post it isn't. –  Vagelford Dec 9 '10 at 9:56

First of all, radiant energy was not light energy to Tesla. He was talking about an entirely different form of energy emitted by the stars. This is evidenced by the fact that this device will produce a charge on a capacitor even at night.

You need to use, as stated earlier, a high voltage capacitor that is of good quality. And it must not be electrolytic. If you use electrolytic then a full wave bridge rectifier using perhaps 1N34A diodes would be needed. But this is not what Tesla used. Read his patents. Also, it needs to be understood what he meant by "amalgamated". This term implies that perhaps he coated his plate with a mixture of metal powders and mercury. Or perhaps he just used a polished plate, probably copper. Experimentation needs to be done in this regard. But don't be fooled by modern physics that state these things are not possible. Much is kept secret. Remember, at one time there were mathematical formulas "proving" that it was impossible to travel at or faster than the speed of sound. Well, they were proved wrong. Look at the SR 71 Blackbird that was developed just ten years after they broke the sound barrier.It could travel at speeds over mach3. Now they say the same about light speed. Who knows when this will be proved wrong.

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Please provide some references 1 that Tesla didn't believe it was light. 2 that he claimed it worked during the night 3 why an electrolytic would need a rectifier but a nonelectrolytic wouldn't. (And where the rectifier would go) 4 that there were mathematical formulas proving that you could not travel faster than the speed of sound –  endolith Jul 7 '13 at 17:21

protected by Qmechanic Jul 6 '13 at 21:08

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