![]() Note how the power supply is wired into the circuit - with its negative end connected to the plate that isn't illuminated. It also serves as a measure of the rate at which photoelectrons are leaving the surface of the photoemissive material. The photoelectric current generated by this means was quite small, but could be measured with the microammeter (a sensitive galvanometer with a maximum deflection of only a few microamps). Keep in mind that this experiment doesn't create electrons out of light, it just uses the energy in light to push electrons that are already there around the circuit. Since the second plate was connected to the first by the wiring of the circuit, it too would become positive, which would then attract the photoelectrons floating freely through the vacuum where they would land and return back to the plate from which they started. Knocking electrons free from the photoemissive plate would give it a slight positive charge. He then illuminated the photoemissive surface with light of differing frequencies and intensities. Lenard connected his photocell to a circuit with a variable power supply, voltmeter, and microammeter as shown in the schematic diagram below. Such a tube is called a photocell (formally) or an electric eye (informally). The tube was then positioned or constrained in some manner so that light would only shine on the first metal plate - the one made out of photoemissive material under investigation. The metal sample was housed in an evacuated glass tube with a second metal plate mounted at the opposite end. ![]() Lenard used metal surfaces that were first cleaned and then held under a vacuum so that the effect might be studied on the metal alone and not be affected by any surface contaminants or oxidation. It was Philipp Lenard, an assistant of Hertz, who performed the earliest, definitive studies of the photoelectric effect. ![]() It meant rebuilding a large portion of physics from the ground up. Repairing this tear in theory required more than just a patch. ![]() When it interacted with electrons, light just didn't behave like it was supposed to. Subsequent investigations into the photoelectric effect yielded results that did not fit with the classical theory of electromagnetic radiation. The era of modern physics is one of completely unexpected and inexplicable discoveries, however. All forms of electromagnetic radiation transport energy and it is quite easy to imagine this energy being used to push tiny particles of negative charge free from the surface of a metal where they are not all that strongly confined in the first place. While this is interesting, it is hardly amazing. Thomson showed that this increased sensitivity was the result of light pushing on electrons - a particle that he discovered in 1897. Hertz found that he could increase the sensitivity of his spark gap device by illuminating it with visible or ultraviolet light. The air gap would often have to be smaller than a millimeter for a the receiver to reliably reproduce the spark of the transmitter. Compared to later radio devices, the spark gap generator was notoriously difficult to work with. In these experiments, sparks generated between two small metal spheres in a transmitter induce sparks that jump between between two different metal spheres in a receiver. The photoelectric effect was first observed in 1887 by Heinrich Hertz during experiments with a spark gap generator (the earliest device that could be called a radio). All electrons are identical to one another in mass, charge, spin, and magnetic moment. This process is called the photoelectric effect (or photoelectric emission or photoemission), a material that can exhibit this phenomenon is said to be photoemissive, and the ejected electrons are called photoelectrons but there is nothing that would distinguish them from other electrons. Under the right circumstances light can be used to push electrons, freeing them from the surface of a solid.
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