Robert Andrews Millikan

“ We do indeed discover new relations, but for the most part the old ones remain. The atomic theory of matter, for example, was not even touched when radio-activity and the divisibility of the atom were brought to light, for nobody who had gone beyond the high school stage in science ever thought of asserting an indivisible atom, and that simply because we had no basis for asserting anything about the insides of the atom. ”

“ We obtain a partial answer to this question when we remember that one of the fundamental assumptions of the kinetic theory is that the energy of agitation of a molecule is determined by temperature alone, and is independent of whether the molecule is large or small. Hence, the energy of agitation of our oil drop ought to be exactly the same as that of one of the molecules of the gas which surrounds it. But this is the quantity which we have just determined experimentally, and which, furthermore, can be computed with great precision from the kinetic theory. ”

“ If you could magnify the air in an ordinary room just a thousand million times, that is, enough to make a good-sized marrowfat pea swell to the size of the earth, yore would see objects about as big as a football—we will not say of what shape, because we do not know anything about it, but they would probably be of the same shape in a given gas—and if the motions would stop long enough to enable us to get a snap shot of the whole situation, you would see on the average one of these objects in a cubical space ten feet on a side. ”

“ Does the triumph or defeat of the kinetic theory of matter or the atomic theory of electricity have anything to do with the practical problems of the modern world? Is anybody going to be better fed or better clothed because of it? ”

“ In order to leave poor old matter, whose independent existence was thus threatened, endowed with its familiar old properties, other physicists of the day preferred to assume that matter in a neutral state shows no electrical properties because it contains as constituents equal amounts of two fluids which they called positive and negative electricity, respectively, and that a positively charged body is one in which there is more positive than negative, and vice versa. ”

“ But since we are here as much concerned with the atomic or granular theory of electricity as with the kinetic theory of matter, let us turn for a moment to consider the present status of our knowledge as to the nature of electricity. Unlike the kinetic theory of matter, the granular theory of electricity can, boast of no great antiquity. ”

“ A drop would often be kept traveling back and forth in the manner described for four or five hours at a time, in the course of which it would change its charge twenty or thirty times because of the capture of ions and the value of each of these different charges would be computed. The beauty and precision of the measurements and the certainty with which the atomic theory of electricity follows from the results obtained can best be appreciated by inserting in full the record of an experiment made upon a particular drop. ”

“ Led by such a bell-wether, the sheep began to jump back over the wall, and the results of that backward movement are still felt in the United States, particularly in high-school texts, despite the fact that to-day the opposition among scientific men to the kinetic hypothesis is absolutely gone, and even Ostwald has admitted his error. Indeed, so direct and so convincing is now the evidence that it is not too much to say that any one who wishes can now have immediate ocular demonstration of the perpetual dance of the molecules of matter. ”

“ Now there can be no doubt that when an electrical charge is placed upon a body, the medium about the body becomes the seat of new forces, and this may be described by saying that the medium about the body has been thrown into a state of strain. ”

“ For while the two possess the same charge, the inertia or mass which, so far as we know, is inseparably associated with a positive unit is that of a hydrogen atom, while that inseparably associated with the negative unit is 1/1.760th as much. The negative units, furthermore, or electrons, are so small in volume and are separated from one another within the atom by so large spaces, that one of them can shoot through hundreds and thousands of atoms without hitting anything or doing anything whatever to these atoms. ”

“ In opposition to that point of view I wish to assert that there are some things, even in science, which we may safely say that we know, that there are some theories which we may be reasonably certain are going to endure—that in fact we may divide the theories of science into three categories, with broad and indefinite lines of division between them, it is true, but yet with real dividing areas, if not dividing lines. ”

“ The practical difference between the two points of view is that in the one case you look for other attributes of the man besides the ability to produce a strain in the bridge, and in the other case you do not look for other attributes. So the strain theory, although not irreconcilable with the atomic hypothesis, was actually antagonistic to it, because it led men to think of the strain as distributed continuously about the surface of the charged body, rather than as radiating from definite spots or centers peppered over the surface of the body. ”

“ This indicates that there are probably no ions which have a very large number of units of excess of one kind of electricity upon them, but it gives us no information as to whether the act of ionization consists in the detachment of only one elementary electrical charge from a neutral molecule, or of two or three; for, so long as the changes are occurring when the field is off, it is impossible to distinguish between the capture of a single ion carrying two or three units of charge, and the successive capture of two or three ions each carrying the unit charge. ”

“ Whether the electrical fluid (or fluids) was supposed to be made up of particles, or to be indefinitely divisible, is not usually stated, though Franklin himself certainly believed in the existence of an electrical particle or atom, for he says: "The electrical matter consists of particles extremely subtle, since it can permeate common matter, even the densest, with such freedom and ease as not to receive any appreciable resistance." ”

“ Furthermore, since it has been definitely proved that an electrical current is nothing but the motion of an electrical charge over or through a conductor, it is evident that the experiments under consideration furnish not only the most direct and convincing of evidence that all electrical charges are built up out of these very units or electrons which we have been dealing with as individuals in these experiments, but that all electrical currents consist merely in the transport of these electrons through the conducting bodies. ”

“ He showed that if a body like one of our minute oil drops is dancing about in a resisting medium subjected to no forces but those arising from its own energy of agitation, that is, from the bombardment of the surrounding molecules, the mean distance which it will drift in a given time, say ten seconds, from its position at the beginning of this time, can be computed in terms of three factors: (1) its energy of agitation, (3) a resistance factor of the medium, and (3) the length of the time interval through which the drift is observed. ”

“ But when it gets out into the open, where the players are scarce, it begins its spectacular flights. Precisely so with the oil drops, and no football game was ever more spectacular or more fascinating than the behavior of one of these oil drops at low pressures. The fact that the motions increase in violence the rarer the gas becomes and the smaller the particles are taken (size being indicated by the speed with which a given particle settles under gravity) is obviously just what ought to happen. ”

“ The first tremendous success of the kinetic hypothesis came about the middle of the nineteenth century, when it gave birth to the principle of conservation of energy, a generalization which grew immediately and inevitably out of the mechanical theory of heat as it lay in the minds of Rumford, Joule and their co-workers. ”

“ Very well, if you are content with that sort of a definition of matter, I will define electricity for you in a similar way and say that electricity is something which is still more fundamental than your atoms of matter since it is a constituent of every one of these hundred different types of atoms which you have been describing. ”

“ About 1900, however, a great stride forward was taken when the atomic hypothesis began to be applied to metallic conductors as well as to solutions, and electrical currents, even in wires, began to be looked upon as due to the transport through the wire of discrete units of electricity, now beginning to be called electrons, these units being either handed on from atom to atom or else being pushed along through the interstices between the atoms. ”

“ Every increase then in man's knowledge of the way in which nature works must in the long run increase by just so much man's ability to control nature and to turn her hidden forces to his own account."
↑ For in the case of bodies moving slowly and uniformly through a resisting medium any two forces produce velocities which are proportional to the forces. The downward force due to gravity is here mg and the upward force due to the field is Fe, in which F denotes the strength of the field and e the charge on the drop. ”

“ It would seem as though the evidence for the kinetic theory were so overwhelming as to convince every type of skeptic except the one whose mental attitude is that of the farmer who had never seen any save farm-yard animals until he went one day to the circus and stood for some moments looking in amazement at the dromedary ”

“ If a biological analogy would make the picture more vivid you can imagine the cages of a menagerie arranged in squares or other regular figures, while the caged animals pace restlessly to and fro between their bars. ”

“ We will consider that there are two kinds of electrification, which we will distinguish by the terms positive and negative, and we will call any body positively electrified if it is repelled by a glass rod which has been rubbed with silk, and we will call any body negatively electrified if it is repelled by sealing wax which has been rubbed with cat's fur. ”

“ Joule, Clausius, Maxwell, Kelvin, Boltzmann—nothing is more surprising to the student brought up in the atmosphere of the scientific thought of the present than the fact that the relatively complex and intricate phenomena of light and electricity had been built together into fairly consistent and satisfactory theories long before the much simpler phenomena of heat and molecular physics had begun to be correctly understood. ”

“ Again, the hypothesis that the earth is a round ball, rotating daily on its axis, and swinging annually around the sun was a few hundred years ago a mere assertion which almost nobody believed; to-day it is an established doctrine which we may count upon to endure, though of course the earth may not always 'keep on doing what it is doing to-day. ”

“ Between 1860 and 1890 the proofs of the kinetic hypothesis came in so rapidly through the brilliant work of such masters as Joule, Clausius, Maxwell, Kelvin and Boltzmann that the scientific world began to be convinced, and only here and there was found a man of standing among the scoffers. ”

“ Assuming it as the basis of our computation there are in a cubic centimeter of gas under normal conditions molecules and the weight of a hydrogen atom is . These numbers can be made more significant to the ordinary reader with the aid of an illustration. If a million men were to be set counting as fast as they could count, say at the rate of 200 a minute, they could count out the number of molecules in a cubic centimeter in just 252 thousand years if none of them ever stopped to eat, sleep, or die. ”

“ The electron is thought by many reputable scientists of the present day to be the primordial thing out of which all matter is built up, so that from this point of view the different atoms of ordinary matter are merely different groupings of these fundamental electrical units. ”

“ Both ultra-violet light and the rays from radium possess, like the X-rays, the power of thus ionizing a gas, and even when no external ionizing agent whatever is at hand, it is found that out of the 27 billion billion molecules which are present in each cubic centimeter of ordinary air, from two tO twenty split up per second into ions. ”

“ Its diameter is about one one-hundred-thousandth of that of the atom. It is the smallest thing we know anything about so far—probably the smallest thing in existence. Such an enumeration of properties is as near to a definition of electricity as we can come now or are ever likely to be able to come. For, since electricity is the most fundamental thing thus far known to us, it is obviously incapable of definition in terms of anything more fundamental. ”

“ It is something too, which like matter is built up out of definite units, but it is unlike matter, in that all of these units are exactly alike so far as we are able to determine, save, however, that a marked difference is found between the positive and negative units. ”

“ Hence a proof was sought which involved no knowledge whatever of either mathematical or physical theory. Fortunately the trail had already been blazed and nothing had to be done but to clear out some of the remaining underbrush which obscured it. It had been discovered as early as 1827 by an English botanist, Brown, that microscopic particles in a liquid keep up incessantly a very minute trembling motion and this phenomenon remained altogether unexplained for more than half a century. ”

“ But, however they may be formed, there can be no doubt of the presence of these ions in the atmosphere at all times, to the extent of from 1 to 15 per cubic millimeter, nor can there be any doubt that it is these atmospheric ions which are responsible for all the manifestations of atmospheric electricity which have been the object of man's awe and worship throughout all ages. ”