Some Contemporary Advances in Physics 



By K. K. DARROW 



Note: Dr. Darrow, the author of the following article, has made it a 

 practice to prepare abstracts and reviews of such recent researches in 

 physics as appear to him to be of special interest. The results of Dr. 

 Darrow's work have been available to the staffs of the Bell System labora- 

 tories for some time and having been very well regarded, it is thought that 

 such a review, published from time to time in the Technical Journal, 

 might be welcomed by its readers. 



The review cannot, of course, cover all the published results of physical 

 research. The author chooses those articles which appear significant to 

 him or instructive to his readers, without attempting to pass judgment on 

 the scientific importance of the different papers published. It is not in- 

 tended that the review shall always assume the same form; at one time 

 it may cover many articles, at another be devoted to only a few, and it 

 may occasionally treat of but a single piece of work.— Editor. 



SOME years ago C. T. R. Wilson of Cambridge University de- 

 veloped a beautiful method for making the paths of moving 

 charged atoms and electrons individually visible. The charged 

 particle flies through a gas such as air, mixed with water-vapor; it 

 ionizes many of the molecules near which it passes; the gas is sud- 

 denly cooled by expansion and the water-vapor is precipitated upon 

 the iomzed molecules, forming a trail of droplets which visibly mark 

 out the path of the ionizing electron or atom. Truly spectacular 

 photographs of such trails, thick straight ones of fast-moving atoms 

 and thin curly ones of electrons, are frequently published in text- 

 bocks and in popular articles. 



The method is now proving very powerful in the study of collisions 

 and close encounters of electrons with atoms and of atoms with atoms. 

 Rutherford having found by another method that the nuclei of atoms 

 are occasionally broken up by unusually direct blows from fast- 

 moving helium nuclei (alpha-particles), the prospect of actually 

 photographing such an important event becomes alluring. How- 

 ever, it is a very rare event; for W. D. Harkins and R. W. Ryan of 

 the University of Chicago photographed eighty thousand alpha- 

 particle trails in air, and only three of the particles struck molecules 

 so squarely as to be deflected through more than a right angle; and 

 of these only one showed indications of having broken the nucleus it 

 struck. This particular collision is shown in Fig. 1 (two photo- 

 graphs of the same encounter taken from different directions at the 

 same moment). In addition to the tracks of the alpha-particle up 

 to and away from the scene of the encounter, there are two more 

 tracks diverging from it, which are probably the tracks of two frag- 

 ments of the struck nucleus. Other interpretations, such as two 

 distinct impacts very near together or a stray radioactive atom 



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