(3) That speeds exactly two times, three times, four times, five times, etc. (always within the limits of observational error --still less than a per cent) could be communicated to the droplet, but never any fraction of these speeds.

He who has seen that experiment, and hundreds of investigators have observed it, has literally seen the electron. For he has measured (in terms of a speed) the smallest of the electrical forces which a given electrical field ever exerts upon the pith-ball with which he is working and with the aid of whose movements he defines electricity itself. Further, he has found that that something which he has chosen to call electricity may be placed upon or removed from his pith-ball only in quantities which cause the force acting upon it either to drop to zero or else to go up by definite integral multiples of the smallest observed force.

counted up to 200 electrons in a given charge his observational error begins to make it impossible to distinguish between 200 and 201; so that the conclusion that large electrical charges are built up in the same manner as are the charges that he can count is of course in the nature of a generalization, but obviously not one of much uncertainty.

But the electron itself, which man has measured in the manner just described, is neither an uncertainty nor an hypothesis. It is a new experimental fact that this generation in which we live has for the first time seen, and which any one who wills may henceforth see.

The measurement of the electron, not as above in terms of the speed that it imparts to a given oil-drop, but in absolute electrostatic units, involved observations of the foregoing sort upon thousands of drops of various sizes, made from a num

ber of different substances, surrounded by a large number of different gases at widely differing pressures, varying from atmospheric down to a millimetre and a half of mercury.

It involved also years of work in finding accurate values of gaseous viscosities, and in determining just how the so-called Law of Stokes must be modified to yield the complete law of fall of a particle through a gas at any density whatever. But all this is only of interest here in showing as Fig. 2 does,* how inevitably all observations on all gases and all substances converge upon the same absolute value of the electron, at the intercept on the e axis of the figure. It is from this intercept that the value of the electron, e=4.774 (‡.005) X10-10 absolute electrostatic units, is directly obtained.

After ten years of work in other laboratories in checking the methods and the results obtained in connection with the oil-drop investigation published from 1909 to 1923, there is practically universal concurrence upon their correctness, despite the vigorous gantlet of criticism which they have had to run.

Electrons, of both the positive and negative variety, are then merely observed centres of electrical force, just as was the charged pith-ball from which we got our original definition of an electric charge, the difference being that electrons are invariable in their charge while pith-ball charges vary because they are built up out of different numbers of electrons. Further, since Rowland proved years ago that electrical currents are simply electrical charges in motion, the proof that electrical charges are built up out of a definite number of discrete electrical particles, electrons, carries with it the proof that electrical currents such as pass through incandescent lamps consist merely in the drifting of immense swarms of these electrons through conductors.

The dimensions of electrons may in general be ignored, i. e., they may both, for practical purposes, be considered as point charges, though, as is well known, the positive has a mass of 1845 times that

This is taken from a repetition of my observations in different gases by my assistant, Doctor Yoshio Ishida. For similar observations upon different drop-substances, see "The Electron," revised ed. 1924, University of Chicago

Press.

of the negative. Why this is so no one knows. It is another experimental fact. It is also well known that we can now count the exact number of positives and of negatives in every atom; that we can assign all of the positives to the nucleus; that we find the negatives scattered partly through the outer regions and partly held within the nucleus; that the number of outer negatives varies from I in hydrogen by unit steps up to 92 in uranium; and that the number of negatives in the nucleus is given by the difference between the atomic weight and the atomic number.

Shall we ever find that either positive or negative electrons are divisible? Again no one knows; but we can draw some inferences from the history of the chemical atom. This is sometimes said by the unthinking to have exploded, but of course every scientist knows that it has never lost an iota of its old reality nor of its old vitality. From an experimental point of view the atom of the chemist was all contained in the facts of definite and multiple proportions in combining powers. For the purposes for which the concept was used, viz., those of chemical combination, the chemical atom is just as much the ultimate unit now as it ever has been.

If

Similarly, it is not likely that the field in which the electron has already been found to be the unit, namely that of atomic structure, will ever have to seek another unit. The new facts which this generation has discovered are certainly the permanent heritage of the race. the electron is ever subdivided it will probably be because man with new agencies, as unlike X-rays and radioactivity as these are unlike chemical forces, opens up still another field where electrons may be split up without losing any of the unitary properties which they have now been found to possess in the relationships in which we have thus far studied them.

The second domain in which, as your award indicates, I have been attempting to take another step, and to assist in bringing the experimental foot up to or even beyond the theoretical, is the field of ether-waves. In this domain I have been seeking since the year 1904 to find some crucial test for the Thomson-Planck

in 1903 to account for two newly discovered experimental facts, viz.:

(1) That X-rays pass over all but an exceedingly minute fraction, say one in a thousand billion, of the atoms contained in the space traversed without spending any energy upon them, but here and there find an atom from which they hurl an electron with enormous speed;

(2) That ultra-violet light has the amazing property, discovered by Lenard in 1902, of ejecting electrons from metal surfaces with an energy which is the same whether the distance of the light is large or small, i. e., which is independent of the intensity of the source.

This Thomson semicorpuscular conception of localized radiant energy was taken up in 1905 by Einstein who, by combining it with the facts of quanta discovered by Planck through his analysis of black body radiation, obtained an equation which, from Einstein's point of view, should govern the intercharge of energy between ether-waves and electrons, viz.: mv2: = hv

p.

It is very easy to see how Einstein arrived at this equation from the conception of the nature of light-waves with which

he started. His fundamental assumption -a very radical one from the point of view of nineteenth-century physicswas that, though light does indeed consist of waves radiating in all directions from the source and waves which may have any frequency v, yet the energy contained in these waves is not spread continuously over the surface of the sphere representing at any instant the socalled wave-front, as is the case in waterwaves or sound-waves, but rather remains concentrated in a definite number of rays or darts of light (light-quanta), the total energy in each dart being proportioned to its frequency and equal to hv, where h is a universal constant connecting frequency and energy.

It is clear that with this assumption, no matter how far one may be from the Source, the energy always available in the light-dart should be hv. If this energy, in being transferred to an electron in a metal, ejects it from the metal with the kinetic energymv2, and if p represents the amount of work done in getting the electron loose from the metal, then it is at once obvious from the principle of the conservation of energy that the energy of the escaping electronmv2 must equal the originally available energy hv, minus the amount p used up in getting it out. This is all that is stated in the foregoing equation.

Now as to the experimental facts: after ten years of testing and changing and learning and sometimes blundering, all efforts being directed from the first toward the accurate experimental measurement of the energies of emission of photoelectrons, now as a function of temperature, now of wave-length, now of material (contact E. M. F. relations), this work resulted, contrary to my own expectation, in the first direct experimental proof in 1914 of the exact validity, within narrow limits of experimental error, of the Einstein equation, and the first direct photoelectric determination of Planck's "h."

In one particular experiment, for example, light of known frequency v, say that of the ultra-violet wave-length 2535 angstroms, was obtained from a quartzmercury arc and a beam of it made to enter a highly exhausted chamber where

=

prevent these electrons from reaching F. This critical potential is the point in Fig. 4 at which the curve expressing the relation between photo-currents and volts for the line labelled 2535 hits the volt-axis. The frequency of the incident light was then changed and the experiment repeated with all the different wave-lengths which are recorded above the curves shown in Fig. 4.

This point of intersection is seen from Fig. 4 to move definitely and unambiguously to the right as the wave-length changes from 2535 through five different stages up to 5461. (The lower half of the figure is simply the right-hand continuation of the upper half.) Fig. V then shows how beautiful and exact a linear relation exists between the critical volts, i. e., the energy of emission, and the frequency,

available at the time these experiments were performed.

This work, like that on the electron, has had to run the gantlet of severe criticism, for up to 1916 not only was discussion active as to whether there were any limiting velocity of emission of electrons under the influence of light, but other observers who had thought that a linear relation existed between energy and frequency had not found the invariable constant h appearing as the ratio. But at the present time it is not too much to say that the altogether overwhelming proof furnished by the experiments of many different observers working by different methods in many different laboratories, that Einstein's equation is one of exact validity (always within the present small limits of experimental error)

proof of a relation which is just the inverse of Einstein's. They bombarded a metal target with electrons of known and constant energy and found that the maximum frequency of the ether-waves (general X-radiation) thereby excited was given, with much precision, by mv2 hv.

D. L. Webster, now at Stanford University, then proved that the characteristic X-ray frequencies of atoms begin to be excited at just the potential at which the energy of the stream of electrons which is bombarding the atoms has reached the value given by hv=mv2, in which is now the frequency of a socalled absorption edge.

In France de Broglie, and in England Ellis, on the other hand, measured with great precision the speed of electrons

large field of so-called ionizing and radiating potentials. This has also involved the utilization and verification of the same reciprocal relation between frequency and electronic energy which is stated in the Einstein equation and which constitutes in its inverse form the cornerstone of Bohr's epoch-making treatment of spectral lines. This work on ionizing potentials all takes its start in Franck and Hertz's fundamental experiments, but the field has been most actively and successfully explored since 1916 in America, especially by Foot and Mohler, Wood, Davis, and Goucher, McLennan, and others.

In view of all these methods and experiments the general validity of Einstein's equation is, I think, now universally con