L. Page — A Century's Progress in Physics. 323 



firmed Lorentz's theory in every detail, and yielded a 



value of — in substantial agreement with that obtained 



for cathode rays. Subsequent research, however, has 

 shown that in many cases more components are found 

 than the elementary theory calls for. Hale has detected 

 the Zeeman effect in light from sun spots, proving that 

 these blemishes on the sun's face are vortices caused by 

 whirling swarms of electrified particles. Recently Stark 

 and Lo Surdo have found a similar splitting up of lines 

 in the spectrum formed by light from canal rays (rays of 

 positively charged particles) passing through an intense 

 electric field. This phenomenon has as yet received no 

 adequate explanation. 



On discovering that an electric current is capable of 

 producing a magnetic field, Ampere had suggested that 

 the magnetic properties of such substances as iron might 

 be explained on the assumption of molecular currents. 

 The electron theory considers these currents to be due to 

 the revolution, inside the atom, of negatively charged 

 particles about an attracting nucleus. It occurred to 

 Richardson that this motion should give the atom the 

 properties of a gyrostat. Hence if an iron bar be rotated 

 about its axis, the atoms should orient themselves so as to 

 make their axes more nearly parallel to the axis of rota- 

 tion. Thus its rotation should cause the bar to become 

 a magnet. Barnett 9 has tested this hypothesis, and has 

 found the effect Richardson had predicted. From the 



strength of the magnetization produced, the value of — 



can be computed. Barnett finds a value somewhat 

 smaller than that for cathode rays, but of the right order 

 of magnitude and sign. Einstein and De Haas have 

 detected the inverse of this effect, i. e., the rotation of an 

 iron rod when it is suddenly magnetized. 



X-Rays. — In 1895, on developing a plate which had 

 been lying near a vacuum tube, Rontgen 10 was surprised 

 to find distinct markings on it. As the plate had never 

 been exposed to light, it was necessary to suppose the 

 effect to be due to some new and unknown type of radia- 

 tion. Further investigation showed that this radiation 

 originates at the points where cathode rays impinge on 



9 S. J. Barnett, Phys. Eev., 6, 239, 1915, and 10, 7, 1917. 



10 W. C. Eontgen, Wied. Ann., 64, 1, 1898 et seq. 



