DEPARTMENT OF TERRESTRIAL MAGNETISM. 289 



insulators and conductors, as required by all theories, the experiment proves 

 that only the electrical fraction (l — K"^) of the dielectric is moving. 



The theory of Hertz is shown to be inconsistent with Maxwell's theory 

 in that it requires, in general, a surface divergence or a volume divergence 

 of the electric displacement when the motion occurs. 



Reference is made to a paper by Swann in which the author's experiments 

 on electromagnetic induction in the case of rotary motion are included among 

 those made to solve the so-called problem of unipolar induction. This is 

 not correct. Indeed, several pages of the original paper were devoted to an 

 attempt (not free from error) to prove that such experiments could not be 

 used for that purpose. The experimental results, as stated in the paper, 

 are consistent with current theory (that of Maxwell and Lorentz) . As to the 

 electrical field in unipolar induction, when a conducting solid of revolution 

 magnetized along its axis is in steady rotation about this axis, it was worked 

 out long ago by Jochmann and later by Larmor, who have given all the details 

 for the case of the sphere. The formulse given by Swann are not new, but 

 so far as they go are identical with those of the early investigators. The con- 

 siderations which Swann has given to show that, on the basis of the electron 

 theory, when a solenoid or an Amperian whirl rotates on its axis, the lines of 

 induction can not be considered to share the rotary motion, were given by 

 Pegram in 1917, together with the theory, also given by Swann, of the author's 

 and other experiments on the basis of Maxwell's equation for the electromo- 

 tive intensity. The theorem used by Swann to investigate the molecular field 

 is also not new, having been given by Maxwell. 



The author's experiments on electromagnetic induction and translatory 

 motion are briefly described and his earlier and later interpretations of one 

 of them are referred to. The latter, whose final acceptance was hastened by 

 a letter from Professor Eddington, is fundamentally identical with an inter- 

 pretation published much later by Swann. Swann gives the mathematical 

 theory by a process unnecessarily complicated, stating that Maxwell's equa- 

 tion for the electromotive intensity can not be immediately applied to show 

 that the field is polar, because the vector potential is not independent of the time, 

 which is incorrect. In this paper the theory for both a steady two-dimensional 

 field and for the actual case of the experiments is given according to Max- 

 well's equation. 



The results of the experiments on rotary and translatory motion are com- 

 pared. 



Progress Report of Committee on Relations between Terrestrial Magnetism, Terrestrial 

 Electricity, and Solar Activity.^ Louis A. Bauer. 



As the result of a preliminary meeting of the Committee on March 4, 1922, 

 at Washington and by correspondence, the following progress report of the 

 Committee on Relations between Terrestrial Magnetism, Terrestrial Elec- 

 tricity, and Solar Activity, was made. 



Mr. Bauer stated that various measures of solar activity have been com- 

 puted at the Department of Terrestrial Magnetism, as based on the following 

 data: Wolf- Wolf er sun-spot numbers, Greenwich sun-spot areas, Greenwich 

 faculse areas, flocculi areas, solar prominences according to the observations 

 at various observatories, especially at Kodaikanal, India, and solar-constant 

 values obtained by the Smithsonian Institution. The director of the Kodai- 

 kanal observatory, Mr. John Evershed, courteously supplied the Department 



1 Presented at the annual meeting of the Section of Terrestrial Magnetism and Electricity of 

 the American Geophysical Union, March 7, 1922. Members of Committee: Louis A. Bauer, 

 chairman; C. G. Abbot, R. L Paris, G. E. Hale, and C. F. Marvin. 



