Comparisons of Magnetic Standards, 1905-14 



271 



ning of its field work in 1905 some standards to which the magnetic elements, observed in 

 all parts of the Earth, could be referred with sufficient accuracy not to vitiate, from a prac- 

 tical standpoint, its published results. Accordingly, after a careful examination of all 

 information available, a first provisional attempt was made to fix " International Standards," 

 and the large ^Mld-Edelmann instruments No. 26 (declinometer, magnetometer, and earth 

 inductor) of the United States Coast and Geodetic Survey, which are installed at its mag- 

 netic observatory at Cheltenham (Maryland), were taken as standards after the following 

 corrections had been applied: for declination and for inclination, O'.O; for horizontal 

 intensity, COOIH.' The latter correction w^as that derived during the period 1902-07 

 from comparisons between the Cheltenham standard and 16 magnetometers of most varied 

 ilesign and make, some belonging to the United States Coast and Geodetic Survey and 

 some to the Carnegie Institution of Washington (Department of Terrestrial Magnetism), 

 the constants for each of these magnetometers having been determined independently. 



Next C. I. W. magnetometer No. 3 was singled out from a number of magnetom- 

 eters and its constants were determined in various ways; e. g., the moment of inertia was 

 determined with different bars, the deflection distances and the distribution coefficients 

 were carefulh' determined, etc. When the constants were established as well as was then 

 possible, it was found that C. I. W. magnetometer No. 3 gave values of H which required 

 a correction of approximately +0.00015// (37 for Washington) on the pro\asional inter- 

 national standard. The Z)-correction for C. I. W. magnetometer No. 3, from limited com- 

 parisons, appeared to be practically zero. Accordingly, the C. I. W. pro%'isional standards 

 for D and H to which all land magnetic observations, 1905-13, given in Vol. I and this one, 

 have been referred, was taken to be C. I. W. magnetometer No. 3, with no correction for 

 decUnation and a correction of +0.00015// for horizontal intensity. 



IMPROVEMENT OF STANDARDS FOR DECLINATION AND HORIZONTAL INTENSITY. 



Let us next examine into the possibility of still further improving the provisional D 

 and H standards. In Table 31 D there are summarized the corrections of the C. I. W. pro- 

 visional standards, as determined in two different ways for the period 1907-14. It should 

 be remarked that magnetometer C. I. W. No. 3, on which the C. I. W. provisional standards 

 were based, has been kept almost constantly at Washington, where, from time to time, field 

 magnetometers have been compared with it. All the many magnetometers used in the 

 comparisons have had their constants independently determined. One might apparently 

 assume that the mean corrections resulting from the data in the table ought not to be far 



Table 31 D. Corrections of the C. I. W. Provisional Standards for Declination and Horizontal Intensity. 



from the truth. It should be stated that in the two sets of data, I and II, account has been 

 taken, as far as possible, of the correction to an observed value of H arising from the effect 

 caused by bending of the deflection bar when the magnet is mounted on it. 



'L. A. Bauer: PreUminary note on an "International Magnetic Standard," Terr. Mag., v. 12, 1907, pp. 161-164. 



