DEPARTMENT OF TERRESTRIAL MAGNETISM. 339 



material in the air undoubtedly go into the type of the slowly moving Langevin 

 ions, the calculated conductivity should be even smaller; it is probably for this 

 reason that the calculated values of n^-]-n_ for the Long Island Sound 

 observations comes out even greater than the observed values. 



The latter portion of the report is devoted to a mathematical discussion of 

 the possibility of determining the nature and amount of active rnaterial in the 

 atmosphere from an analysis of the decay curves for the active wire. It 

 appears that the customary method of drawing conclusions as to the nature of 

 the products in the atmosphere by comparing the decay curves for a wire 

 exposed thereto with that of a wire exposed to emanation contained in a small 

 closed vessel, is not justified. The activity curves are analyzed in the report, 

 use being made of the theory of radioactive disintegration, and it is found that 

 while some of the curves can be explained by radium emanation alone, others 

 require the presence of a product of longer decay period than radium A, B,or 

 C. The possibility of this extra product being a product of thorium emana- 

 tion, as is generally assumed to be the case on land, is discussed by the author. 



An attempt to calculate the actual amount of radium emanation in the air 

 directly from the theory of the Elster and Geitel method, without assuming any 

 empirical relation, results in a much smaller value for the emanation content 

 than that given by the empirical relation, unless it is assumed that the average 

 specific velocities of the active carriers are much smaller than is generally 

 supposed. 



The origin and maintenance of the Earth's charge. W. F. G. Swann. Read at the special 

 meeting on atmospheric physics of section B of the American Association for the 

 Advancement of Science, San Francisco, August 5, 1915. Terr. Mag. vol. 20, 

 105-126 (September 1915). 



The paper consists of two parts. Part 1 is devoted to a general discussion 

 of certain broad principles which must be considered in the formulation of any 

 theory of atmospheric-electric phenomena, and to a consideration of former 

 theories. In part 2 a new hypothesis is provisionally formulated, and its 

 consequences are traced. 



Part 1 commences by considering the possibility of a general circulation in 

 the atmosphere by which the negative electricity flowing upwards at one place 

 is conducted down at some other place. It appears that such an explanation 

 is untenable, for no regions have been discovered presenting the phenomenon 

 of a continual return current, and apart from this fact, the existence of a 

 circulation of the kind depicted would necessitate that the electromotive forces 

 around closed paths, in which the flow was taking place, would have to be of 

 the order of magnitude of 10^ volts. Electrostatic forces can contribute nothing 

 whatever to a line-integral around a closed circuit. A consideration of such 

 values of the line-integral as could be obtained on the basis of the change of 

 magnetic induction due to the Earth's magnetic field through a closed circuit, 

 or of the motion of the Earth and atmosphere in the magnetic lines of force of 

 the Earth, shows that, apart from the circumstance that they would be of a 

 nature unsuitable to correspond to the facts, they would be of an order of 

 magnitude entirely too small to play any appreciable part in the phenomena. 



The various possible types of hypotheses which may be made to account for 

 the maintenance of the Earth's charge are capable of being grouped under 

 three heads: (1) We may imagine that negative electricity is fed into the 

 Earth from the outside in some unspecified manner. In this case it mil be 

 necessary to assume that the vertical conduction current is dissipated again 

 into space. (2) We may imagine that negative electricity is supplied con- 

 tinuously to the Earth and positive electricity to the atmosphere at all places. 



