Who Determined the Charge of an Electron?

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The charge of an electron was determined by Robert Millican and a group of scientists in 1909 with the oil drop experiment. Listen to a description of the famous oil drop experiment with insight from a math and science teacher in this free video science lesson.

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Video Transcript

Hi I'm Steve Jones and I'm going to tell you who determined the charge on an electron and how he did it. Well it's a physicist from the United States, Robert Millican, in cooperation with a group of scientists there and in 1909 he devised this experiment. The experiment was very important because the charge on an electron is a very fundamental thing. It is unit charge. It is one charge. It happens to be one negative charge and there is a balancing positive charge so we have got positive and negative charges. So what he did in this experiment he enabled scientists to get a very fundamental value. The value eventually was worked out to be 1.602 X 10 -19 coulombs. It doesn't really matter what the value was but how did he do it? It is a very famous experiment. The oil drop experiment and what he actually did he took a device which actually you need a microscope to do this and you have a microscope set at the side here and the microscope looks across a section here where you have got two plates. They are usually round plates with a whole through the middle of the top one and through the top hole you can drop droplets of oil. Now these droplets are not big. You use an atomizer that is to make oil spray and when you spray it over the top the oil droplets pop through there. Now those oil droplets actually are not electrons but they are charged when you atomize them and they're charged with one or two or three or four electronic charges. So what you can do because they are charged you can actually let them fall through to the bottom plate and you can time the fall and this is a viscosity formula that tells you how much force you have to apply to stop the particle and you can move these particles once it has gotten to the bottom it has a negative charge. By increasing the voltage here you can make it go back up to the top and then let it fall again so in this way you can observe through the microscope the speed at which this particle falls and from that you actually believe it or not determine its radius, the radius of the particle and you know then from that you can work out the charge on the particle. Once you know the charge on the particle well you know it is a multiple of electronic charges. This is the fundamental. You cannot have half of it. You can't have one and a half so if you find that you get three on there or five on there or six on there you will find that it is all a combination of a number of these charges. If you do it with enough different particles you can determine the charge on the particle and of course it was Robert Millican who did that way back in 1909 nearly 100 years ago.


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