"The strangest lunacy that ever was," I said. "It must be."

"It would be," said Adams, "if it were lunacy. But it's something else." "What then?"

VOL. LXXVII.-6

"That's for you to guess."

The best that Adams could do was to ease the man's final confinement. He died that spring, worn out by the incessant repetitions of his ordeal. The

73

Sometimes it is one foot which is put forward first, sometimes the other, but continuous progress is only made by the use of both-by theorizing and then testing, or by finding new relations in the process of experimenting and then bringing the theoretical foot up and pushing it on or beyond, and so on in unending alternations.

The terms of this year's award state that it is given "for work on the fundamental electrical unit and on photoelectricity." In both fields my own work has been that of the mere experimenter whose main task has been to devise, if possible, certain crucial experiments for testing the validity or invalidity of conceptions advanced by others.

The conception of electrical particles or atoms goes back a hundred and seventy years to Benjamin Franklin, who wrote, about 1750: "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."

This theoretical conception was developed in no little detail by Wilhelm Weber in papers written in 1871. The numerical value of the ultimate electrical unit was first definitely estimated by G. Johnstone Stoney in 1881, and in 1891 this same This is the Nobel Address, hitherto unpublished, delivered in Stockholm, May 23, 1924, with additions and amplifications.

physicist gave to it the name "the electron."

In 1897 the experimental foot came forward with J. J. Thomson's and Zeeman's determinations of the so-called ratio of charge to mass-by two wholly distinct methods. It was these experiments and others like them which in a few years gained nearly universal acceptance among physicists for the electron theory.

There remained, however, some doubters, even among those of scientific credentials, for at least two decades-men who adopted the view that the apparent unitary character of electricity was but a statistical phenomenon. And as for educated people of the non-scientific sort, there exists to-day among them a very general and a very serious misconception as to the character of the present evidence. A prominent literary writer recently spoke of the electron as "only the latest scientific hypothesis which will in its turn give way to the abracadabra of to-morrow."

It is perhaps not inappropriate then to attempt to review to-day as precisely as possible a few features of the existing experimental situation and to endeavor to distinguish as sharply as may be between theory and some newly established facts.

The most direct and unambiguous proof of the existence of the electron will probably be generally admitted to be found in an experiment which for convenience I shall call the "oil-drop experiment." But before discussing the significance of that advance I must ask you to bear with me while I give the experimentalists' answer to the very fundamental but very familiar query: "What is electricity?" His answer is naïve and definite. He admits at once that as to the ultimate nature of electricity he knows nothing.

He begins rather with a few simple and familiar experiments and then sets up some definitions which are only descriptions of the experiments and therefore involve no hypothetical elements at all. He first notes the fact that a pith-ball or a bit of paper, after contact with a glass rod that has been rubbed with silk, is found to be endowed with the new and striking property so that it tends to move away from the rod with a surprisingly strong and easily measurable force. He describes that fact, and affirms at the same time his ignorance of all save the existence of these forces, by inventing a

possible test of the correctness or incorrectness of Franklin's conception of a particle, or an atom, of electricity it was clearly necessary to reduce the charge on the pith-ball to the smallest possible amount, to change that charge by the most minute possible steps, and then to see whether the forces acting upon it at a given distance from the glass rod (i. e. in a constant field) have any tendency to increase or decrease by unitary steps.

The success of the experiments first performed in 1909, was wholly due to the design of the apparatus, i. e., to the relation of the parts.

new word and saying that the pith-ball has been put into a positively electrified state, or simply has received a charge of positive electricity. He then measures the amount of its charge by the strength of the observed force.

Similarly he finds that the pith-ball, after contact with an ebonite rod that has been rubbed with cat's fur, is attracted, and he proceeds to describe this experiment by saying that it has now received a charge of negative electricity. Whenever the pith-ball is found to have been put, by contact with any body, or by any other process, into a condition to behave in either of the foregoing ways, it has by definition, received a charge of either positive or negative electricity. The whole of our thinking about electrical matters starts with these two simple experiments and these two definitions.

In order now to get the most crucial

The pith-ball itself which was to take on the smallest possible charge had of course to be the smallest spherical body which could be found and yet which would remain of constant mass; for a continuously changing gravitational force would be indistinguishable in its effect upon the motion of the charged body from a continuously changing electrical charge.

A non-homogeneous or non-spherical body also could not be tolerated; for the force acting on the pith-ball had to be measured by the speed of motion imparted to it by the field, and this force could not be computed from the speed unless the shape was spherical and the density absolutely constant. This is why the body chosen to replace the pith-ball was an individual oil-droplet about a thousandth of a millimetre in diameter blown out of an ordinary atomizer and

kept in an atmosphere from which convection currents had been completely removed by suitable thermostatic arrangements. The glass rod, the purpose of which was to produce a constant electrical field, was of course replaced by the two metal plates C and D (Fig. 1) of an air condenser, one of the plates (D) being attached to the positive, the other (C) to the negative, terminal of a battery, and a switch being added as shown in the figure so as to make it possible to throw the field on or off at will.

In order to be able to measure very accurately the force acting upon the charged oil-droplet, it was necessary to give it about a centimetre of path in which the speed could be measured. This is one of the most important elements in the design, the overlooking of which has caused some subsequent observers to fall into error. The centimetre of path and the constancy of field then fixed the approximate size of the plates, the diameter of which was actually twenty-two centimetres. They were placed 16 mm. apart.

The field strength, too, about 6,000 volts per cm., was vital and new in work of this kind. It was the element which turned possible failure into success. Indeed, nature here was very kind. She left only a narrow range of field-strengths within which such experiments as these are all possible. They demand that the droplets be large enough so that the socalled Brownian movements* are nearly negligible, that they be round and homogeneous, light and non-evaporable, that the distance be long enough to make the timing of the rate of motion of the droplet accurate, and that the electrical field be strong enough to more than balance gravity by its upward pull on a drop carrying but one or two electrons. Scarcely any other combination of dimensions, field strengths, and materials could have yielded the results obtained. Had the electronic charge been one-tenth its actual size, or the so-called "sparking potential" in air a tenth of what it is, no such experimental facts as are here presented would ever have been seen.

The observations which gave an unam

Rapid, irregular motions of agitation of very minute particles in liquids or gases which are actually due to the bombardment that these minute particles experience from the flying molecules which surround them.

biguous answer to the question as to the atomic nature of electricity consisted in putting a charge upon the drop, in general by the frictional process involved in blowing the spray, letting the charged drop drift slowly down through a pinhole in the centre of plate C into the space between C and D, in then measuring both its speed of fall under gravity when the electrical field was off and its speed of rise against gravity when the electrical field was on, and then in repeating these measurements after the charge on the drop had been changed in a considerable number of different ways; for example, by ionizing the air just beneath it by alpha, beta, or gamma rays from radium, by illuminating the surface of the drop itself with the ultra-violet light, by shooting X-rays both directly at it and beneath it, etc. The results of these changes in charge, as is now well-known, and as is shown in particular cases in the accompanying table, were: