It is not possible, in the present article, to give more than a superficial idea of the way in which the reality and general properties of the negative corpuscle have been established. The strength of the corpuscular theory lies in the fact that it has been tested from many angles, and that the results obtained by approaching it from the most diverse viewpoints have, in the main, harmonized with one another astonish ingly. Doubts that may be felt with regard to the legitimacy of the assumptions made in any one line of investigation tend to lose their force when confronted by cumulative evidence from widely different sources. It is true that incon sistencies and other difficulties have developed here and there in connection with the corpus cular theory, but that could only be expected, because the entire subject is still new, and progress in the application of the theory has doubtless been retarded and distorted to a con siderable extent by the persistence of certain of our older conceptions and postulates that are no longer defensible, but to which we still cling because we have not yet learned wherein our error lies. In the main, the data that have been obtained are singularly consistent. Moreover, the corpuscular theory has proved to be ex traordinarily rich in its suggestiveness, and has led to many lines of investigation that have been fruitful and productive of good results. This alone would justify us in following it still further, to see where it will ultimately lead.
Prominent among the quantities that we should like to determine in connection with the negative corpuscles of which we may for the time being assume the cathode ray to consist, are the following: (1) The mass (m) of a corpuscle, (2) the electric charge (e) that it bears, and (3) the speed (u) with which the corpuscle is moving under given conditions. Let us see how these magnitudes were first • obtained: It has long been known, from the general theory of electricity, that a charged particle, when moving in a magnetic field and at right angles to the lines of magnetic force, is de flected so that it tends to describe a circular arc (instead of a straight line), in a plane pernen dicular to the direction of the magnetic lines. It is, in fact, a simple matter to show that when the charged particle is moving freely in space, its charge per unit of mass (denoted in symbols by the ratio bears to its velocity the same ratio that the reciprocal of the radius of the circle in which it moves bears to the intensity of the magnetic field that causes the path to be circular. Now it is easy enough to subject the cathode ray to the action of a magnetic field, and the deflection of the cathode ray thus pro duced is quite marked even when the field is not very strong. The radius of the circular arc that is described by the ray in a field of known strength is also measurable without any special difficulty, and hence we can determine, with a fair degree of precision, the ratio of — to u.
This, however, is only one step in the solution of the problem, for we do not yet know either or fs, separately. Some experimenters, assuming that the ratio of the charge on the particle to the mass of the particle is the same in the cathode ray as it is in the case of the ions that are involved in electrolysis, sub stituted this value and then proceeded to deter mine, by means of the experiment just cited, the value of u,— that is, the speed of the par ticles in the cathode ray. By this means a value of u was obtained that was not greatly different from the speeds •appropriate to atoms of matter.) This result was illusory, however, because the fundamental assumption that in is the same in the cathode beam as it is in elec trolysis was wholly gratuitous, and was also, as the event proved, entirely wrong.
Wiechert succeeded in measuring the speed of the cathode-ray particles directly, by means of an exceedingly ingenious apparatus, which, although it is apparently incapable of giving results of any high order of precision, is at least competent to show the order of magni tude of the speed, and hence to check the va lid:ty of assuming that it is similar to the ordinary molecular speed, or that the ratio in the cathode-ray particles is the same as it is in the ions that are concerned in electrolysis. His method depends upon the deflection of the cathode ray by a magnetic field, but he used two magnetizing coils, energized by a rapidly alter nating current having a period commensurate with the time required by the cathode-ray par ticles to traverse a considerable length of the tube. The cathode was placed at one end of the tube and at the other end was a fluorescent screen, which, by its luminosity, showed where the ray came in contact with it. Between the cathode and the fluorescent screen two d:a phragms were placed, so that the ray was wholly intercepted except for a small part that could pass through a central perforation in each diaphragm. The first magnetizing coil was placed between the cathode and the first dia phragm, and as the alternating magnetic field that it produced varied, the cathode ray vi brated to and fro across the surface of the first diaphragm. The apparatus was so adjusted that the ray passed through the opening in this diaphragm only when the magnetic field pro duced by the coil was at its maximum in one particular phase — the oscillating beam being then at the extreme end of (say) its upward swing. At this moment the ray would pass
through the opening in the first diaphragm, proceed down the tube to the second diaphragm, pass through the central opening in this, and then register itself by producing a round, luminous spot in a fixed position on the fluores cent screen beyond — the alterations in the magnetic field being so rapid that the spot ap peared steady, although the illumination was really intermittent, because the cathode ray, since it could pass the first diaphragm only when at the extreme upward part of its peri odic sway, traveled down the tube in a series of spurts or pulsations. The second magnetiz ing coil was placed at or just beyond the sec ond diaphragm, and in the absence of a special adjustment or relation (to which we shall presently refer) the alternating magnetic field produced by this second coil, acting upon the cathode ray as it massed the second dia phragm, would again deflect it, and cause it to impinge upon the fluorescein screen above or below the spot at which it would strike if the second coil were absent or inactive. It is evi dent, however, that if the magnetic field of the second coil were always hi the zero phase when the cathode-ray pulsation reached it, there would be no second deflection produced, and the luminous spot on the screen would occupy the same position that it would have if the sec ond coil were absent. With the apparatus dis posed as described, it was Imown that the magnetic field of the first coil was at its maxi mum phase when the cathode ray passed through the first diaphragm, and (if the sec ond coil did not displace the luminous spot on the screen) it was also known that the magnetic field of the second coil was at its zero phase when the cathode-ray pulsation reached the second diaphragm. In performing the actual experiment the two magnetizing coils were made identically alike and were placed in the circuit in parallel and with symmetrically arranged leads, so that the phase of the current at any given instant would be the same in each. The magnetizing current was fumished by a modified Tesla high-frequency coil, provided with a pair of condensers of lmown capacity; and from the known electrical constants the frequency of the magnetic oscillations in the two fields could be calculated. The experiment then consisted in determining the shortest dis tance by which the two magnetizing coils could be separated, consistently with the second one having no effect. (We say the ashortest dis tance' because it is evident from the nature of wave-motIon that a similar zero effect would be observed whenever the time of transit of the cathode ray from one field to the other hap pened to be one-fourth, three-fourths, five fourths or any odd number of fourths, of the time of a complete period of the current in the magnetizing coils). In one experiment this least distance was found to be 39 centimeters, and the number of complete oscillations of the magnetic field, from either coil, was found to be 32,000,000 per second. Hence the dine re quired for the cathode beam to travel 39 centi meters, in this case, was the 128,000,000th part of a second. Therefore its speed was 4,992, 000,000 centimeters per second—or, to express it in the usual way, and to as high a degree of precision as the data will warrant, 5.0 X 10' centimeters per second. (The symbol 10' stands for the ninth power of 10. In the same way 10—' stands for the reciprocal of the ninth power of 10. A notation of this Icind is in common use in physics for expressing large numbers, as it avoids the use of long rows of ciphers, which are not only confusing to the eye but are also likely to lead to error from misreading, or from the accidental addition or omission of ciphers in copying or printing). It is evident from the foregoing result that the speed of the cathode ray corpuscles is of an entirely different order of .magnitude from the usual speed of trans lation of gas molecules. The average molecular speed in hydrogen gas, for example, at atmos phenc pressure and at the temperature of melt ing ice, is only about 17 X 10' centimeters per second. Chi the other hand, the velocity of fight, in a vacuum, is about 3 X 10' centimeters per second, so that the velocity of the cathode ray particles, in this experiment, was about one sixth of that of light, or about 30,000 times as great as the speed of translation of hydrogen molecules. It should be understood that no great degree of accuracy is claimed for the particular numerical result just quoted, and it should also be understood that the speed of the cathode-ray particles varies considerably with the degree of exhaustion in the tube, and with the intensity of the electric field in the vicinity of the cathode. It is evident, however, (1) that we are here dealing with speeds en tirely transcending anything previously known in connection with the translatory motion of matter, and (2) that Crookes was in all proba bility right when he expressed the view that cathode-ray phenomena bring us in touch with matter (if indeed these particles are °matter° in the ordinary sense) in a very different state from any with which we have had previous experience.