330 CARNEGIE INSTITUTION OF WASHINGTON. 



Concerning the origin of the Earth's magnetic field. L. A. Bauer. (Presented before the 

 Philosophical Society of Washington, October 14, 1916.) 



The various recent theories regarding the origin of the Earth's magnetic 

 field were reviewed with particular reference to their bearing on the general 

 topic: the constitution of the Earth's interior. The hypothesis of chief 

 interest in this connection, namely, that of an iron core being the cause of 

 terrestrial magnetism, has inherent in it many difficulties, which, however, 

 may not be insuperable. Should experiments decisively show that increased 

 pressure elevates the critical temperature of magnetization, then the depth of 

 10 to 12 miles, now supposed to limit the presence of materials in the magnetic 

 state, would be increased. HoAvever, the few experiments available indicate 

 that increased pressure lowers the critical temperature of magnetization. 



The various hypotheses as to the Earth's magnetic field being caused b)' 

 electric currents within the Earth's crust, or that the field is connected in some 

 manner with the speed and direction of rotation of the Earth, were briefly 

 discussed. The exceedingly small effect to be observed renders conclusive 

 laboratory experiments, if not a hopeless task, certainly a very difficult one 

 with present apphances. 



The author reiterated a belief, already expressed on a former occasion, that 

 our chief hope at present of determining the origin of the Earth's magnetic 

 field, appears to be in the direction of determining what causes the field to 

 vary in the remarkable manner it does. The definite limitations imposed by 

 the Variations in the Earth's magnetic field, both of the periodic and aperiodic 

 kind, and the departures of the field from the simple uniform type, are too 

 frequently overlooked by theorists. Most theories, for example, are found 

 inadequate when the attempt is made to explain, besides the origin of the 

 field, the secular variation as it is actually observed. 



In conclusion it was pointed out that the solution of some of the questions 

 entering into the problem of the origin of the Earth's magnetic field must be 

 deferred until the completion of the magnetic survey of the Earth now in 

 progress under the auspices of the Carnegie Institution of Washington. 



On the improvement of the direct-recording declinograph. W. G. Cady. 



The report covers the experimental work' done since 1909, and describes 

 the additional improvements in design and theory of the direct-recording 

 declinograph.2 The record made by the instrument was found to be affected 

 by temperature effects caused chiefly by expansion and contraction of the 

 horizontal brass arm from which the magnet was suspended; this effect has 

 been eliminated by using, instead of the single supporting-post, two posts. 

 The ink-reservoir formerly carried by the pen has been made fixed and the 

 butt end of the pen bent down to extend into the fixed reser^^oir; the weight 

 of the pen is thus made constant and the balance of the movable pen is 

 improved. The difficulty arising from jarring of the rocker-arm cariying the 

 record-paper has been eliminated by introducing a dash-pot with a specially 

 devised by-pass. A winding device has been added to automatically reel up 

 the record-paper as it comes from the instrument. 



The record-paper heretofore used has been replaced by a paper received 

 from the Leeds and Northrup Company of Philadelphia; the new paper 

 practically does away wi th the blurring of the slow-dr^dng ink previously 



'The work was doue by Professor Cady at Wesleyan University, Middletown, Connecticut, with 

 the cooperation of the Department of Terrestrial Magnetism. 



»For description of the instrument and of its theory see Terr. Mag., vol. 9, pp. 69-80. 1904, and 

 vol. 11, pp. It5-l.=i2, 1900. 



