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JUST as the accurate measurements of controlled phenomena have taught us much that is definite and much that is useful about the divisibility of matter, so also cleverly designed and skillfully executed experiments are giving us equally definite knowledge of the atomic nature of electricity, of the fact that there is a measurable and seemingly ultimate limit to the divisibility of an electric charge, and what the exact magnitude of the ultimate charge is. Like nearly every other discovery of importance, this one, too, has a long and an honorable history. In fact it can be traced back, step by step, almost a century. The first, and in many ways most fruitful contribution to this subject, was made by that prince of experimenters, Michael Faraday. It consisted in proving that when an electric current is passed through a water solution of any one of certain substances, the substance itself is taken out of the solution, to an extent dependent entirely upon the quantity of electricity so passed, and upon the nature of the substance dissolved. Thus, when the same current is passed in series through the water solutions of several salts, such as sodium chloride, silver nitrate, copper sulphate, and the like, the weight of the metal deposited, or, under proper conditions, permanently removed from the solution, is directly proportional to the atomic weight of the metal itself, and inversely proportional to its valency, that is to say, to the number of hydrogen atoms necessary chemically to take the place of one atom of the metal. This proportionality between the quantity of el'2ctricity passed through the solution, and the resulting amount of chemical decomposition, holds rigidly true, within the limits of experimental error, under all conditions, and hence it seems practically certain that to each ion in a solution conveying an electric current there belongs a definite electrical charge; and that the smallest of such charges is that carried by a single hydrogen atom, or by a single atom of any other univalent substance. A bivalent atom, such as copper in copper sulphate, carries just twice the smallest, or univalent charge; a trivalent atom three times the smallest charge; and so on for atoms of still higher valency. Hence, in electrolytic solutions there is a measurable smallest possible charge of which larg ?r electrolytic chargps are only definite multiples. A natural inference from these experimental facts would be (and this inference was drawn by many) that electrical charges are just as definitely atomic in their nature as, for instance, is a mass of iron. That just as, under given conditions, there is a limit to the actual, though not to the conceivable divisibility of matter, so too there is a limit to the actual, though again nnt [c the conceivable divisibility of an eledrh charge. This, however, was only an inference, and for many years the way to test it, in the case of any quantity of electricity o:.her than that used in the decomposition of an electrolyte, was not evident. Besides, even in the process of electrolysis, the most refined measurements could directly detect nothing less than the aggregate of countless millions of elementary charges, so that the value of the unit charge was only inferen-tially and not immediately determinable. About a dozen years ago, J. J. Thomson, H. A. Wilson, and C. T. R. Wilson began, in the Cavendish laboratory at Cambridge, England, a series of most ingeniously devised and skilfully executed experiments that proved the existence of minute electrical charges in conducting gases, and showed their average value to be, as nearly as could be determined, the same as that of the electrolytic charge spoken of above. C. T. R. Wilson showed that a fog of water particles will form in dust-free air whenever the degree of super-saturation is sufficiently pronounced. If the air is ionized, or has been rendered conducting through the action of X-rays, or otherwise, then a four-fold Buper-saturation causes condensation of the water vapor on the negative electrons; a six-fold super-sat- uration gives condensation on the positive electrons; and an eight-fold on the neutral molecules of the gas itself. Now, a knowledge of the amount of water vapor present, and of the extent of the cooling below the dew point by which the super-saturation is produced The apparatus by means of which Prof. Millikan has isolated an ion and measured its charge. enab1es one easily to compute the weight of the water condensed as fog. Furthermore, if the fog is left to itself, it slowly settles at a rate which, as Stokes proved long ago, depends upon the size of the individual particles and upon the viscosity of the medium through which they fall. A measurement, then, of the rate at which the fog falls, since it all falls at about the same rate, enab:es one to calculate the size of the individual particle, and this knowledge of the siz- of the particle, together with a knowledge of the amount of water condensed, at once gives the total number of particles. On bringing a charged body near this electrified fog its motion is modified. and a means is at hand for meas-uring the magnitude of the charge on each partic'e. PrObably the simplest method of measuring this charge, through the behavior of the fog as a whole, is that devised by H. A. Wilson. The rate of fall of the fog is measured when there is no external electric feld act'ng on it, and thus the size and weight of the individual particle determined. After this, a vertically [ljrected uniform electric field is brought to bear on the particles, and regulated to just counteract the force of gravity, so that the fog neither riSES nor falls. Under these conditions of equilibrium the value of the charge on each particle of fog, multiplied by the s'rength of the field, is equal to the weight of the suspended particle, and hence ,vhen both the weight of the droplet and the strength of the field that keeps it in suspension are known, the numerical value of tlle cllarg3 is also known. All this, however, assumes that the rate of fa'l of the fog en ma ss e, the group rate, is th8 same as would be that of a single or.e of its droplets if alone. This, as a 1 tter of fact, is not rigidly true. For this, and for other 1eaSOIs too, 1: se3med extremely desirable to Prof.' R. A. Mi1l!l'an, of the Ryerson physical lOlbo:atory, at the University of Chieago, somehow to iso:ate and to measure an ion entirely by itself. A f
