The optical projection of the tracks of alpha particles

THE OPTICAL PROJECTION OF THE TRACKS OF ALPHA PARTICLES .* BY

H. T . PYE. THE well-known expansion apparatus due to C . T . R . Wilson, admirable as it is for photographic purposes or for visual use by a few observers, does not lend itself readily to projection so that a large audience may see the tracks of a-particles . The same is . i. Ft(-,

true of the Shimizu modification of Wilson's apparatus . Reflectoscope methods have so far not proved practicable . The writer has devised a modification of the Wilson-Shimizu apparatus so that light from an arc lamp passes through the expansion chamber and the shadows of the tracks are clearly shown on a screen by the usual projection methods . This apparatus in its first immature stages was originally shown to many members of the British Association at the Macdonald Physics Building at McGill University early in August_ It has since received one notable improvement . At first the axis of the cylindrical expa nsion chamber was horizontal and * Communicated by Dr . A . S . Eve, Associate Editor, Vor .. rg8, No . rr88-g8

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the clouds, falling under gravity, appeared to move upwards on the screen as soon as they were formed . By placing the axis vertical, and using a plane mirror for reflection at 45°, this undesirable feature has been removed . If the active deposit of radium is placed at the end of a thin rod so as to be in the middle of the expansion chamber, then the appearance on the screen is somewhat similar to Fig . 3, which, as a matter of fact, is a snapshot of the rays in the expansion chamber itself . The tracks are not straight because the ions and droplets had begun to disperse, and because convection currents

were present in the chamber ; also there was too much radium on the tip of the rod . Various improvements are still in progress, but it will be seen that the general scheme is that of the Shimizu apparatus, and that many of C . T . R . Wilson's suggestions have been found of great value . Fig . i is a photograph of the actual apparatus and the main scheme is indicated in Fig . 2. C is a hollow brass cylinder with a plate glass cover . C' is a hollow brass piston (also with a plate glass cover) to This which a reciprocating motion is given by the crank C' . crank gives a constant stroke of ii mm ., but in order to vary the expansion ratio the cylinder C can slide in the ways 1V and and IV' by means of the screw S. The piston is driven by a one-eighth horsepower electric motor through a pinion of five leaves and a gear of 120 teeth, giving a maximum of 70 strokes a minute, which of course can be reduced by a rheostat in circuit if desired .



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TRACKS of ALPTJA PARTICLES .

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The cylinder glass is well insulated from the rest of the apparatus, while the surfaces of both of the inner faces of the glass plates must be made conducting with a mixture of clear gelatine and dissolved copper sulphate . following C . T . R . Wilson's recommendation, A difference of potential of 200 volts between these surfaces is used to remove the clouds after their formation on the ions caused by the respective a particles . The expansion chamber is sealed tight with rubber washers and screws, Fir,. 3 .

after some moisture has been introduced, the piston being then at the inner end of its stroke . It is inadvisable to use radium because the emanation (radon) causes contamination . Hence radium C, or polonium, or thoriurn C is more suitable . If fine needles are introduced, facing one another, connected to a Wimshursst machine, it is also possible to project the spark discharge effects at various potentials and distances . An arc lamp of about r5oo candlepower is sufficient, but a water or alum cell is placed near the lantern condenser in order to cut down the heat entering the expansion chamber ; also to prevent the lubricant, vaseline and beeswax, between cylinder and piston from a tendency to " run " if too warm . In the demonstration from which the photograph of Fig . 3 was taken, the projection on the screen was about six feet in diameter when the apparatus was about thirty feet away, and in



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a room well darkened the shadows of the tracks were visible to a distance of about forty feet or more . I have to thank Mr . S . Amesse for the photographs . NOTE BY DR . EVE .

This apparatus when well developed is likely to have a large and important vogue for popular audiences and for instructional purposes ; possibly it may have its uses for research work also . As the scheme for projection was entirely worked out by Mr . Pye, I suggest that the apparatus he called the " H . T . Pye Projection Modification of the C . T . R . Wilson-Shimizu Apparatus ." The projection on the screen is excellent and much superior to that indicated by Fig . 3 . The Internal Composition of the Earth . ROKURO Y AMAMOTO . (Ann . de Phys ., May, June, 1924 .)-In endeavoring to find what is the character of the depths of the earth there are two chief ways to proceed . The first is that of the geologist who, like a physician making a diagnosis, from appearances on the surface infers the condition of the interior . The second is that of the seismologist who seeks information from the only messenger that traverses the interior of our planet, the wave coming from an earthquake . In his own country of Japan this seismologist has had all too much occasion to learn of these waves at first hand . " I had opportunity," he says, " at the time of the great catastrophe that ravaged Japan on September 1, 1923, to verify my formula whose justification seems to me to be established." With fine literary art, with a knowledge derived from observation and with strong feeling attained through bitter suffering, he sketches in a page an outline of the meaning of earthquakes to the human race, and makes his readers feel his sincerity when he states, " It is not, however, with the sole end of establishing more accurate results than those hitherto possessed that I have undertaken this work . To combat effectively an evil, it is necessary before all else to recognize its manifestations, and every piece of work that can some day aid us in protecting mankind against these sudden convulsions of the globe seems to me to be of unquestionable usefulness ." Wiechert, in 1897, proposed his theory that the earth has a central core of iron, so that somewhere between the surface and the centre the density of the earth has one or more discontinuities of value . In this paper the basis of this theory is scrutinized and fundamental errors are discovered . The author therefore rejects the conception of an earth with sudden changes of density, confirming his conclusion by an appeal to celestial mechanics which has long found difficulty in reconciling Wiechert's theory with the facts of notation and precession . G. F. S.