68 ELECTROMAGNETIC RADIATIONS AND MATTER 



THE STRUCTURE OF MATTER 



The Elementary Particles and Atomic Architecture 



Some of the key experimental facts accumulated within a few years of 1900 

 illustrate the bases upon which our knowledge of structure depends. 



Roentgen found that his unknown, or "X," rays would cause fluorescence 

 in zinc sulfide and barium platinocyanide; and further that they would 

 ionize gases and darken a photographic plate. They were therefore easily 

 detected by an electroscope, or by an increase in current through a gaseous 

 discharge tube, or by photographic techniques. He studied penetration 

 through paper, wood, and metals, and showed that difference in penetration 

 is one of degree rather than of kind (cf. the quotation which opened this 

 Chapter.) 



A fluorescent screen on each end of a cylindrical gaseous discharge tube 

 showed that particles, presumably charged, pass between the electrodes in 

 each direction. By placing metal shields between positive and negative elec- 

 trodes, and by impressing a voltage between horizontal plates placed with 

 their plane parallel to the direction of flow, it was shown that the rays com- 

 ing from the positive electrode bend toward the negative horizontal plate, 

 and are therefore positively charged; and likewise the rays from the negative 

 plate bend toward the positive plate, and are therefore negative. The nega- 

 tive particles were called cathode rays, and positives canal rays. 



In 1897, J. J. Thomson (not William Thomson, Lord Kelvin) measured 

 the deviation of the (negative) cathode rays in an electric and magnetic field, 

 and obtained a value for the quotient of the charge to mass, i.e., e/m. This 

 value was found to be the same (1.757 x 10 H cou/g) no matter what ma- 

 terials were used. Cathode rays were therefore recognized as elementary 

 particles of matter, and were called electrons. The (positive) canal rays, how- 

 ever, were found to be different for different materials. 



By an ingenious experiment in late 1897, Milliken was able to obtain an 

 independent measure of e, the charge on the electron. One or two electrons 

 were trapped on atomized oil particles, and the electrical force necessary to 

 prevent each oil particle from falling under the influence of gravity was 

 measured. Since the size of the particle could be determined from the rate 

 of free fall, the charge absorbed by the particle could be evaluated. The 

 smallest value obtained, 4.78 x 10~ 10 electrostatic units (1.600 x 10~ 19 cou), 

 corresponded to one electron absorbed. 



From Thomson's value of e/m, the mass could then be determined as 

 9 x 10" 28 g. This was an astounding achievement, the fact that exact meas- 

 urement of this mass was possible by these means, whereas the most sensi- 

 tive chemical balance weighs to only approximately 10" 6 g! 



For the canal rays, e/m for H + was found to be 1820 times smaller than 

 for the electron. Faraday in 1830 had shown by electrolysis that the charge 



