Millikan Oil Drop Test


Oil-drop experiment (Oil-drop experiment), is Robert Millikan and Harvey Fletcher (Harvey Fletcher) in 1909 A physics experiment conducted in 1988. Robert Millikan thus won the Nobel Prize in Physics in 1923.

The purpose of this experiment is to measure the charge of a single electron. The method is mainly to balance gravity and electricity, so that the oil droplets are suspended between two metal electrodes. And according to the known electric field strength, the total charge of the whole oil drop is calculated. After repeated experiments on many oil droplets, Millikan found that the total charge value of all oil droplets was a multiple of the same number, so it was determined that this value was the charge of a single electrone:


Experimental device

Milligan set up a uniform electric field by arranging two metal plates in parallel in a horizontal manner, as two poles, a considerable potential difference can be generated between the two poles . There are four small holes in the metal plate, three of which are used to inject light into the device, and the other is equipped with a microscope for observation experiments. The oil droplets sprayed into the plate can be changed by controlling the electric field.

In order to avoid the increase of error caused by the evaporation of oil droplets due to light irradiation, this experiment uses oil with a lower vapor pressure. A small number of the oil droplets get an electric charge due to friction with the nozzle before being sprayed into the plate, and become the test object.

The flaws of the experiment

Richard Feynman once described "Cargo cult science" in a graduation speech at the California Institute of Technology in 1974. Mention:

From past experience, we have learned how to deal with some self-deception situations. For example, Millikan did an oil drop experiment to measure the charged amount of electrons and get an answer that we know today is not quite right. His data is a bit biased because he used an inaccurate air viscosity coefficient value. Therefore, if you sort out the data obtained by measuring the electrical charge after Millikan, you will find some very interesting phenomena: draw these data and time as a graph, and you will find that the ratio of the value obtained by this person Millikan's value is a little bit larger, the next person gets a bit larger, and the next one gets a bit larger, and finally, it stabilizes when it reaches a larger value.

Why didn’t they discover that the new value should be higher in the first place? ——This incident makes many related scientists blush ashamed——Because obviously many people do things like this: when they get a result higher than Millikan's value, they think something must be wrong, and they will desperately look for it. And found the reason for the error in the experiment. On the other hand, when the results they obtained were similar to Millikan's, they would not be so careful to review. Therefore, they excluded the so-called material that was too different from each other and disregarded it. We are now well aware of those tricks, so we will never commit the same problem again.

60 years after Millikan’s oil drop experiment, historians discovered that Millikan had published 58 observations, and he himself had made 140 observations. In the experiment, he passed pre-estimation and removed those data that he thought were biased and error-prone.

External link

  • (Chinese) Simulation experiment animation-Millikan oil drop experiment

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