JOURNAL OF CHEMICAL EDUCATION
11. WHY DO WE BELIEVE IN ELECTRONS? ANTHONY STANDEN ' The Interscience Encyclopedia, Brooklyn, New York S T u n m T s have heard so much about electrons in popular literature that they believe in them with passionate and unshakeable faith. It is the business of the science teacher to explain the experimental basis for our belief in electrons. But what experiment, or group of experiments, is most suitable? An earlier paper [THIS JOURNAL, 23, 356 (1946)] showed that J. J. Thomson's experiment with cathode rays, in which elm was measured, is not suitable for this purpose, for the phenomena it presents are just as easily accounted for without the electron as with it. In this paper Millikan's oil-drop experiment is considered. Millikan's oil-drop experiment is a beautiful demonstration that unit charges of electricity exist, all equal to one another within experimental error, and of the magnitude 4.77 X lo-" e. s. u. It is thus remarkably convincing evidence of the e5stence of minute particles --of something-hut examination will show that we cannot, from this experiment alone, conclude whether the particles are particles of electricity or particles of matter. For we have no way of telling whether that which is picked up by an oil drop is an electron or an ion, and we already have evidence that electricity, when intimately associated with matter to form ions, occurs in discrete unit charges. The laws of chemical combmation, and several other lines of evidence, give us excellent ground for believing in atoms of matter. If we accept the atomic hypothesis, then Faraday's laws of electrolysis lead us to the conclusion that, in electrolytic conduction, electricity becomes associated with matter in a discontinuous fashion, each atom or group of atoms taking up 1, 2, or 3 or more equal units of positive or negative electricity. Millikan's oildrop experiment supports this conclusion, and adds to it in three ways: (1) it gives a beautifully perspicuous demonstration of the existence of the minute particles which theory demands; ( 2 ) it shows that ions in gases carry unit charges just as ions in electrolytic solutions do; and (3) it shows that the electric charges,
unlike the masses of the atoms themselves, are all equal within experimental error. In other words, there are no L'isotopes" of electricity. But it does not give evidence of the existence of particles of electricity apart from charged atoms. In metallic conduction, in a "space charge," or in cathode rays, is electricity to be regarded as continuous or particulate? The oil-drop experiment, the atomic theory, and Faraday's laws are entirely compatible with the suggestion that electricity in these three forms is continuous and that the "atomicity of electricity" observed in electrolysis and in gaseous ions is a property not of electricity but of matter. The suggested theory is this-electricity is essentially coutinuous, but it is a property of atoms of matter to become associated with electricity only in discrete amounts. Those who like mechanical analogies in the 19th-century manner can imagine atoms provided with little cups, all of the same size, which would either be empty or completely full of electricity. At the pole of an electrolytic cell, the ions would discharge their cupfuls, and the electricity would m u together, forming a continuous fluid, and proceed as such along the metallic conductor. We would then he quite unjustified in concluding the independent existence of the electron from the oil-drop experiment, for we might be making an error similar to that of an imaginary being, whose experience of water was limited to watching a firemen's bucket chain, and who should conclude "water has a grossly particulate structure; it never occurs in less than a bucketful." Millikan's oil-drop experiment, then, is not entirely satisfactory as evidence for the independent existence of the electron, for it is too easily explained without the assumption that electricity, as well as matter, has an inherent particulate structure. If students are to be given training in the careful weighing of evidence and not merely indoctrinated with an unshakable faith in current theories, the problem of the experimental evidence for electrons requires very careful consideration.
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