320 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



tively charged satellites revolving round the nucleus as repre- 

 sented in Fig. 23. Imagine the region in Fig. 23 to be a mag- 

 netic field directed towards the reader. The effect of such a mag- 

 netic field would be to push outwards on the satellite a, thus 

 increasing its periodic time of revolution, whereas the effect would 

 be to push inwards on the satellite a', thus decreasing its peri- 

 odic time of revolution. Now the hypothesis which has been 

 used in the discussion of the Zeeman effect is that a given line of 

 the spectrum of the hot vapor is due to the rotation of a certain 

 satellite in the atom, at a certain speed. The plane of the orbit 

 of this particular satellite has every possible orientation in the dif- 

 ferent atoms of the vapor as shown by A, B and C t Fig. 23, 

 and, when the vapor is not in a magnetic field, the periodic time of 

 rotation of the satellite a is the same for all the atoms. When, 

 however, the vapor is in the magnetic field, the periodic time of 

 the satellite a is increased, the periodic time of satellite a' is 

 decreased, and the periodic time of satellite a", the plane of 

 whose orbit is parallel to the magnetic field, is unaltered. There- 

 fore, instead of one single spectrum line corresponding to the given 

 satellite, there will be three lines, one in the original position and 

 one on each side of the original position. 



24. Lippmann's electrometer.* A pool of mercury underneath 

 an electrolyte, such as dilute sulphuric acid, can of course be 

 used as an electrode of an electrolytic cell. When this is done 

 the surface tension of the mercury is altered, the change of sur- 

 face tension being approximately proportional to the polarization 

 electromotive force (electromotive force between the metal and 

 the electrolyte). This change of surface tension of mercury by 

 electrolytic polarization may be demonstrated by the change in 

 level of a mercury column in a capillary tube when the surface 

 of the mercury column is polarized. This effect was discovered 

 about 1870 and it was employed by Lippmann in the construction 

 of a capillary electrometer in which the movement of a mercury 



* See Wiedemann, Die Lehre -von der Elektricit&t, Vol. II, pages 708-720, for a 

 full discussion of the polarization of mercury. 



