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|Topic: Dr. Abram's Electron Theory
William F. Hudgings
Section: Part VI, Radium Analysis
Table of Contents to this Topic
| Next, an analysis was made of the radium emanations
themselves to ascertain their exact nature. Although these radiations
are invisible to the eye nevertheless they were made to appear visible
by various ingenious contrivances. By placing a small quantity of
radium into a prepared cavity in a solid lead block, the rays were then
permitted to pass through a tiny aperture in the lid and made to graze
along a wall which had been covered with a phosphorescent substance
such as zinc sulphide. The radium rays brushing against this wall
produced a faintly illuminated streak. Now a magnet of known strength
was held against the wall a few inches to one side of the streak.
Immediately the streak divided into three parts, one portion bending
toward the magnet another portion bending away from the magnet, while a
third portion retained the original position, being neither attracted
or repelled. This proved that three distinct streams are emitted, one
of which consists of positively charged particles, another consists of
negative particles, while another is of such a nature as to absolutely
defy the magnetic field. The positive stream they called Alpha rays,
the negative stream Beta rays and the independent stream Gamma rays,
after the first three letters of the Greek alphabet. These are usually
referred to by the Greek characters themselves, as a, b and y rays.
By letting these rays fall squarely upon a phosphorescent target held at variable distances from their source, minute sparks, plainly visible through a telescope were produced by the a and b streams thereby proving that they consist of infinitesimal particles like leaden shot from a shotgun. The y rays were found to be of the nature of X-rays but far more penetrating.
After determining the existence of the three different kinds of emissions from radium various experiments were then undertaken to determine their exact nature. First their respective velocities were ascertained. This was accomplished by noting the amount of deflection that could be produced by magnets of known strength acting upon the charged particles. Prof. Schuster proposed a mathematical formula by which velocity may be computed where the amount of curvature and the strength of the magnetic field are known. But his equations required certain assumptions and were therefore not entirely satisfactory until later experiments supplied the missing data. Accordingly Profs. Thomson, Wilson and others experimentally determined the velocity; energy and charge of both the a and b particles without indulging in any assumptions whatsoever.
It was found that an electric current flowing from negative to positive possesses all the properties of the b rays from radium, except velocity; hence it was possible to make a very close study of b particles under most favorable circumstances in vacuum tubes. Such currents are called Cathode rays, because they flow the negative pole (or cathode) to the positive pole (or anode). By boring a hole through the centre of the anode some of the current or ray would pass directly through it because of its velocity, and when it thus fell upon a phosphorescent screen behind the anode the same scintillations were produced as in the case of the radium emanations, thereby showing that the cathode current or stream actually consists of multitudes of individual particles (electrons) which were found to be identical with those which comprise the b rays from radium.
It was arranged that the negative particles (electrons) which pass through the anode would be deflected by a magnet and caused to fall into an insulated hollow vessel. An electrometer connected to the vessel was therefore able to record the aggregate charge of the particles collected within a given time. By a similar arrangement their aggregate energy was measured by means of a galvanometer. With these quantities known, together with the amount of curvature produced by a given force, a simple algebraic equation then yielded the information sought, viz., the velocity. In like manner the velocity of any current or any radioactive emanation may be definitely ascertained. This experiment, and various others of a more elaborate nature, enabled scientists to determine the following facts about radioactive radiations: a rays are almost identical with helium atoms, having a mass 7,000 times greater than b particles. An a particle is not, therefore, an individual proton, but is an aggregation of protons and electrons in which the protons predominate and thus give it a positive charge. These a particles, because of their relatively larger size, are far less penetrating than are the b particles. They are unable to pass through an ordinary sheet of paper. Their velocity is about 20,000 miles a second.
b rays consist of individual electrons which are, of course, negatively charged. They have a mass of about 1/1700th of an atom of hydrogen (the smallest atom known) and can easily penetrate thin sheets of aluminum or iron. They have a maximum velocity of 170,000 miles a second, which is over nine-tenths the speed of light itself.
y rays consist in nature to X-rays but have greater penetrating powers, and like the latter they possess the same velocity as ordinary light, viz., 186,000 miles a second. They carry no electric charge and therefore cannot be deflected by a magnetic or electrostatic field. They are emitted only by those radioactive substances which also emit b rays (some substances emit only a particles). They do not consist of particles such as make up the a and b streams, but are pulsations of energy evidently produced by the atomic "explosion" when the b particles are shot forth. Owing to the unusual penetrabillity of the y rays it is difficult to utilize them efficiently in the study of radio-chemical effects. There remains much yet to be determined regarding these powerful y rays.
Prof. Thomson's apparatus, mentioned above, proved that b particles carry a considerable charge; yet their mass was so excessively minute that it could not be measured by any means then employed. He estimated that it would take a century to collect a weighable amount of electrons in his insulated vessel, viz., one-thirtieth of a milligram. Of course the mass, and consequently the size, of these electrons could have been mathematically computed from their aggregate charge, energy and known speed, provided the number of particles constituting the aggregation were known. Being unable to determine this by means of the aforementioned apparatus, he undertook an entirely different experiment and was happily rewarded with success. The details of this brilliant experiment are quite interesting.
MEASURING AN ELECTRON
By means of a vacuum pump the residual gas in a glass jar was rarefied, and then a stream of electrons (cathode rays or beta rays) was passed through it. The bombardment of the swiftly traveling electrons against the gaseous atoms caused the latter to be deprived of some of their planetary electrons, thus leaving them out of balance. Such atoms are then said to be charged, because they will react either negatively or positively depending upon whether they have too many protons for the remaining electrons, or vice versa. Charged atoms are called ions , and the process of disrupting the atoms by a bombarding current is called ionization. An ion, in other words, is an atom or molecule which is charged by virtue of an inequality in the number of positive and negative electrons which compose it.
The apparatus was so arranged that a known amount of vapor could then be forced into the jar containing the ionized gaseous atoms. Immediately this was done the vapor began to condense around each ion, thereby forming tiny globules of mist or fog. Now it is well known that vapor cannot condense into droplets without a nucleus upon which to form. An uncharged atom or molecule will not permit of condensation around it, but ions readily attract the vapor and cause condensation upon their surfaces, in much the same way that dust does. It is generally because of minute particles of dust in the atmosphere, acting as nuclei for condensation, that we have cloud formations, resulting in fog, mist, rain, rail, etc., upon the earth. These nuclei are frequently so infinitesimal that they cannot, be seen through a microscope, yet in a test tube they may be filtered out with cotton wool to such extent that no condensation can take place when the temperature is lowered to the dew point, even though the air in the tube be supersaturated with vapor. But immediately dust particles are introduced condensation into fog begins. The uncharged molecules of air in the tube are too small to act as condensation centres; something larger like a dust particle, which consists of millions or billions of atoms, is necessary. But the electric tension within an ion seems to compensate for the minuteness of its circumference; hence ions will permit mist to form around them even though the tube be free of dust.
COUNTING FOG-DROPS AND ATOMS
The purpose of the experiment was to ascertain the number of ions present, which, together with their known aggregate charge would enable the mathematician to determine their individual charge, inertia, mass and size. Then, by deduction, the mass and size of the missing electron (which caused the ion to become charged) could be ascertained. But how was the number of ions to be arrived at? Was this done by simply counting the globules of fog in the jar? Exactly; but it was not so simple as it might at first appear. While the ions were present in great number the resulting for appeared as a dense cloud, and the individual globules were of course indistinguishable. But by first rarefying the gas in the jar and then, repeating the ionization and condensation processes the nuclei were fewer in number and the globules were correspondingly larger. They were still too small, however, to count in an ordinary manner, being about 30,000 to the cubic centimeter in the most successful experiment undertaken; and a further rarefying of the ions was found not to aid but really to hinder the success of the experiment.
It was therefore necessary to determine the amount of water in each fog-drop and divide it into the total quantity of water that had been introduced into the jar in vapor form, all of which had now been condensed into fog. The result gave the number of individual globules in the cloud. But how could the amount of water in each tiny fog-drop be ascertained? Even if one of them could be isolated it would certainly be too minute to be measured or weighed by any ordinary process. Nevertheless these tiny droplets have weight and are acted upon by gravitation the same as anything else. Therefore, by noting the rate of their fall their size could be determined mathematically inasmuch as the density of the medium through which they were falling was known.
All clouds are acted upon by gravity; and the larger the globules which compose the cloud the faster the cloud will fall. Sometimes a cloud condenses sufficiently to cause the globules to fall quickly in the form of rain. If the globules have not condensed so as to be as large as raindrops then their fall is slower, in the form of a fine mist; or possibly they settle down still more slowly, in the form of a thick fog. But if the globules are excessively minute they will remain on high as cloud; yet they are not beyond the force of gravity. Contrariwise each globule, no matter how infinitesimal, is slowly but surely falling through the resisting air. Air currents may carry the cloud upward more quickly than the globules are falling downward; but no matter how high and how rapidly the cloud is elevated by the wind, the globules continue to trickle down through it as fast as the resisting medium will permit.
Since, therefore, the size of the globules affects the rate of their fall, large drops falling more rapidly than smaller ones through the same resistance, it has been possible to work out a formula by which the size of any drop or globule may be determined by taking into consideration its velocity of fall and the density of the air through which it falls. In the experiment in question the gradual descent of the cloud in the jar could be easily observed and timed by illuminating the top surface of it with a transverse beam of light and then noting how long it took for it to fall an inch, which was about ten minutes. Then by a simple computation the size of each globule, and the total number of globules was ascertained. This gave the number of ions present, because each globule had an ion as its central nucleus.
Before the vapor was introduced into the jar all the positive ions had been electrically eliminated so that only the negative ions were utilized as nuclei for the vapor. The number of globules therefore gave the number of negative ions only. These were now attracted to an anode which was introduced into the side of the jar, and their aggregate charge and energy measured. Then by dividing the number into the total, the charge and energy of each ion was established. This gave the charge for an individual electron because the ions were charged by virtue of having lost one electron by bombardment. Another simple calculation gave the mass and inertia of an electron. It is important to note that the same value was obtained for the mass of an electron, no matter whether the bombarding stream consisted of beta rays or cathode rays or electronic streams set up by X-rays or ultra-violet light falling upon negatively electrified plates. The ions behaved identically regardless of how they were produced, thus justifying the conclusion that each of the methods accomplished the same thing, viz., shaking lose one electron from each ionized system.
The mass, inertia and charge of an electron now having been mathematically determined, another equally simple calculation established its size, because it is known that a given amount of electric charge, having a given inertia, must exist on a sphere of certain radius and could not occupy a sphere of any other size without doing violence to proven and established electromagnetic laws. The diameter of an electron was thus found to be one-fifth of a trillionth of a centimeter. It would thus take 13 trillion electrons, laying side by side in close formation, to make a row one inch long. Our minds can scarcely comprehend the magnitude of such a number. If we had a book containing 13 trillion pages, and each leaf was as thin as the paper used in this pamphlet, our volume would be 400,000 miles thick -- nearly twice the distance from the earth to the moon.
Inasmuch as the sizes and masses of electrons, atoms, etc., are so excessively small when considered in terms of the customary scientific units such as the centimeter and the gram, the scientist finds it necessary to make a modification in our decimal system; otherwise his figures would become unwieldy. The exigency is easily met, however, by simply using successive products of 10 (for all numbers above 1) and indicating these products by a small figure in the upper right-hand corner, called an exponent, thus:
101 means 1 x 10 or 10
102 " 10 x 10 " 100
103 " 10 x 10 x 10 " 1000
104 " 10 x 10 x 10 x 10 " 10000
105 " one million
1012 " one million billion
1018 " one billion billion
For numbers smaller than 1 it is equally simple. Any fraction may be expressed in negative power of 10 by placing a minus sign before the exponent, thus:
10-1 means .1 or one tenth
10-2 " .01 one hundredth
10-3 " .001 one thousandth
10-6 means .000001 or one millionth
10-9 " .000000001 one billionth
10-12 " .000000000001 one trillionth
Any desired quantity, large or small, may easily by expressed by this system. If, for example, we wish to set down one fortieth of a millionth, instead of writing it .00000025 we would express it as 2.5 x 10-3 (i.e., two and a half times a hundred millionth). Accordingly, instead of specifying the diameter of an electron as one fifth of a trillionth of a centimeter we would designate it as 2 x 10-18 cm. (i.e., two times a tenth-trillionth). Likewise the diameter of the hydrogen atom is written as 10-8 cm. With this method of numerical expression it is as feasible to work out a problem involving excessively minute or excessively large quantities as it is to calculate the number of square yards in a city block. To the mathematician it is but a step from the infinitesimal radius of an electron (10-18 cm.) to the illimitable distance from earth to the farthest nebulae (1024 cm.); yet when these figures are converted into the units of everyday usage they are of such proportions as to appear to the layman as but a fantasy of scientific imagination, and he hastily concludes that they signify nothing more than an elaborated bit of arbitrary guesswork.
The diameter of an atom is exceedingly large in comparison to the diameter of an electron. We would therefore suppose that an atom is made up of billions or trillions of electrons, were it not for the fact that experiments have shown that such is not the case. If all the electrons in any atom were crowded close together they would comprise only a small fraction of the atom's bulk. The hydrogen atom, for instance, contains but one electron and one proton. This was demonstrated by Profs. Thomson, Aston and others in a set of brilliant experiments in which they actually succeeded in isolating the proton from the electron and measuring it. They bombarded hydrogen gas with positive rays, causing the electron to be separated from its nucleus. The nucleus was then deflected by a magnetic field of known strength and the amount of deflection from a straight line was registered upon a photographic plate. From the amount of deflection produced by the magnet, the mass and inertia of the particle were computed. Its inertia was found to be 1845 times that of an electron, and thus its mass is almost equal to the entire atom itself; although it is evident that its size is identical with that of an electron. Inasmuch as they are elemental charges of electricity, the one positive and the other negative, and the two are complementary to each other. Equality in size does not signify equality in mass. Various other experiments have corroborated the fact that greater portion of the mass of all atoms resides in the nucleus.