RESULTS OF COMPARISONS OF MAGNETIC STANDARDS, 1905-1914. 



By L. A. Bauer and J. A. Fleming. 



EXPLANATORY REMARKS. 



This part of the jjresent volume contains, first, the results of the various intercompari- 

 sons of magnetic standards obtained by the observers of the Department of Terrestrial 

 ISIagnctism, from lOOo to 1914, inclusive, the world over, both at magnetic observatories 

 and in the field among themselves. Secondly, there will be found a preliminary summary 

 of the results of intercomparisons in recent years by others, and an exhibit of the relation 

 of these results to those of the Department. Preliminary results of some of the comparisons 

 have been pubhshed in Terrestrial Magnetism and Atmospheric Electricity, vol. 14, pp. 3-16, 

 1909; vol. 16, pp. 61-84, and pp. 137-162, 1911; and in Land Magnetic Observations, 

 1905-1910, p. 43. A provisional summary, up to 1910, was prepared for the meeting of the 

 Magnetic Commission of the International Meteorological Conference held at Berlin in 

 September 1910, and printed in the Report of the Proceedings. All previous results are 

 superseded by those in the present report. 



When a general magnetic survey of the globe is to be executed on a common and con- 

 sistent plan, it becomes a matter of importance to know how far instrumental constants and 

 reductions to standards, as determined at one place, can be relied upon in other places where 

 the magnetic elements are considerably dilTerent, or what changes may be expected during 

 strenuous field campaigns, such as must be carried out in more or less unexplored countries. 

 If, furthermore, the magnetic results obtained by various organizations, using instruments 

 of greatly different construction, are all to be reduced to a common basis, it becomes increas- 

 ingly important to have the requisite data at hand for the proper correlation of all work. 



It has been foimd that, for one reason or another, magnetic instruments may differ 

 among themselves by quantities far exceeding their observational errors. Sometimes these 

 differences can be referred to defective constants, at other times they are due to causes 

 inherent in the instruments themselves. Thus, for example, referred to a standard which, 

 according to elaborate tests, appears to give values of the horizontal intensity, H, of an 

 absolute accuracy on the order of 1 in 10,000, the //-corrections, as will be seen later, for 

 17 magnetic observatories over the Earth are within 0.0005//, some being plus and some 

 minus. For 15, the correction reaches 0.0005// to 0.001//, and even more quantities 

 approaching or even exceeding the errors of good field-instruments. 



WTienever circumstances permitted, the method of intercomparisons of magnetic instru- 

 ments described in Vol. I, pp. 19-20, was followed. Only occasionally did it happen that, 

 for lack of time or other reason, the full program had to be curtailed. 



Disturbed sites are avoided for the intercomparisons of instruments, but this can 

 not alwaj'S be done in the field, as for example, in the ocean work where islands, or ports, 

 often afford the only opportunity for the desired comparisons. If the preliminary examina- 

 tion has shown the existence of pronounced local magnetic disturbance, and if another site 

 is not available, it is arranged that, at the same station, the magnetic systems of the 

 various instruments are in the same horizontal plane. Should this procedure not be pos- 

 sible, then the height of magnet from a suitable reference point, e. g., from the top of a stake 

 driven into the ground, is carefully noted and determinations are made at each station to 

 find the necessary corrections for the various levels in which the intercomparisons had to 

 be secured. With these precautions, it has been found that results of sufficient accuracy for 



field work can be obtained. 



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