Milikan's Oil Drop Experiment

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Onion the Omniscientist: Furthermore, Thompson hypothesized from his conclusions “that the mass of a single electron was at least 1000 times smaller than that of the smallest atom."

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Clueless Cat: Is that the closest he got? It sounds pretty vague to me. 1000 to infinity is an infinite number of possibilities...
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Onion the Omniscientist: Yeah... However, it was originally his experiments that were modified to find an electron's charge, which could then calcuate the exact mass of an electron with his ratio of charge to mass (qe ÷ m = 1.76 X 1011coul/kg). After 20 years, when Robert A. Milikan, changed his “suspension measurement” from water vapor to oil droplets, it became known as “Milikan's Oil Drop Experiment.”
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Clueless Cat: I take back my words! So Thompson came up with “Thompson's Vapor Drop Experiment!”
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Onion the Omniscientist: Well, okay then! But even though it was a slight improvement, it was totally necessary because the water droplets would evaporate too quickly and therefore using oil was much more accurate than many other scientists, since he also decided to use single drops rather than the usual bunch.
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Clueless Cat: Ha! Sounds very interesting, could you educate me about this wonderfully important experiment?
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Onion the Omniscientist: Of course! Oil droplets would be sprayed from an atomizer (perfume bottle) into a chamber, which might pass through a pinhole at the bottom of the positive electrode, which is a metal plate. He then waited for a droplet to reach terminal velocity, which is when the force of a lag on a falling object is the same as the force of the gravity, which means it will continue to fall at the same pace or even slow it down. By connecting a battery to the electrodes, it created an electric field between them. He also irradiated the chamber with x-rays, which caused the air particles to lose electrons and become cations (positive ions).
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Clueless Cat: Oh!... I bet I'll be a positive ion, but you wouldn't!
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Onion the Omniscientist: Well, I'll be an anion for sure! Anyways, some of the particles that captured one or more electrons would be negatively charged so that it would have floated upwards or fall even slower, since its negative charge attracts it to the positive electrode, rather than falling down to the negative plate due to gravity. By adjusting the voltage of the electric field to suspend the particle, he could find the charge of an electron by setting the force of the gravity equal to the amount(s) of charge necessary to suspend the particle(s). His results showed that all the drops had a charge that was a multiple of the 1.6 X 10-19 coulombs, which he inferred to be the charge of an electron (e). To find the mass of an electron, he simply put e over the charge to mass ratio, qe ÷ m, which equals .091 X 10-30 kg. Which means that the mass of an electron is 1,837 times smaller than the smallest atom!
If you thought that was exciting, read on about the Rutherford-Bohr model
 

by Nancy Huang & Ivy Ngo