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Special Reports 



to 0.000457.fl r . That the change in the moment of inertia has been linear with time 

 during the period of field service of the instrument is evidenced by the compilation of 

 the differences on I. M. S. in horizontal intensity obtained from the comparisons at 

 Washington, and given in Table 20A. 



Table 20A.- Corrections on I. M. S. in Horizontal Intensity for C. I. W. Magnetometer No. 19. 



See p. 7. 



It is to be noted that the difference on I. M. S. for magnetometer No. 19 is less than 

 the value previously adopted. This improvement results (1) from the adoption of a 

 value of the distribution coefficient P' resulting from all the data available since the 

 construction of the instrument, and (2) from an improvement in the absolute value of 

 the moment of inertia for inertia-bar No. 19, the bar used for both determinations of 

 inertia of magnet 19L and its stirrup. The mean value of P' adopted for the constants 

 of March 31, 1916, is +7.71 instead of +7.60 the value previously used; this change 

 required corrections of +0.00010# and of 0.00010m to the values of horizontal inten- 

 sity, H, and magnetic moment, m, computed on the basis that P' was +7.60. The 

 original value of the logarithm of the moment of inertia for inertia-bar No. 19 was too 

 small by 0.000279; this difference requires that additional corrections of +0.00032/? 

 and 0.00032?w be applied to values of H and m determined using the original constants 

 of the instrument. 



As a result of the discussion, constants of date March 31, 1916, have been adopted 

 for use with magnetometer No. 19 for all observations prior to March 31, 1916; values 

 of H and m computed by those constants require the corrections on account of the 

 linear change in the value of the moment of inertia for magnet 19L and its stirrup. 



C. I. W. magnetometer No. 25 was used at the shore stations of the Carnegie. Its 

 performance is typical of what may be expected even under the most difficult conditions 

 provided reasonable care is used. The long magnet, 25L, with its stirrup also showed 

 a pronounced change in its moment of inertia while in field use. There was no change 

 in the moment of inertia of magnet bL and its stirrup for the second magnetometer. 

 No. 5, used by the Carnegie party (see Table 20E). Thus through the comparisons 

 made at shore stations between Nos. 5 and 25 we have an excellent series for investi- 



" The results with C. I. W. magnetometer No. 19 during September 1912 to December 1913 (there were no field observa- 

 tions during January to February 4, 1914, the latter being the date on which the instrument was returned to the office) are 

 already published in Volume II using a correction on C. I. W., on the basis of the constants of April 9, 1915, of +0.00039// 

 which is equivalent to +0.00024// on I. M. S. The average correction on account of the change in the moment of inertia for 

 magnet 19L and its suspension during September 1912 to December 31, 1914, to be applied to values of H computed by the 

 constants of April 9, 1915, is 0.00030//. The published results, therefore, are given for an average corrected value of (C. 

 I. W. C. I. W. No. 19, constants of April 9, 1915) = +0.00069/i/ or are equivalent to (C. I.W. C. I. W. No. 19, constants 

 of March 31, 1915) =+0.00027//; the final corrected value adopted for (C. I. W. C. I. W. No. 19, constants of March 31, 

 1910) = +0.00035//, which leaves an outstanding difference of only 0.00008//. It is, therefore, not worth while to correct 

 any of the results already published in Volume II, Res. Dep. Terr. Mag., for C. I. W. magnetometer No. 19. 



