REDUCTIONS TO STANDARD INSTRUMENTS. 

 MAGNETIC STANDARDS ADOPTED. 



The Department's extensive intercomparisons of magnetic instruments at 

 Washington, in the field, and at magnetic observatories in all parts of the Earth 

 have made it possible to refer its data to provisional "International Magnetic 

 Standards." Such data obtained prior to 1914 were discussed in detail in Volume 

 II, pages 211 to 278; the corresponding data obtained during 1915 to 1920, which 

 will be given later in a special report, bear out, in general, the conclusions reached 

 in Volume II. The "International Magnetic Standards," as stated, are provi- 

 sional, particularly for intensity, pending the completion and intercomparison of 

 absolute instruments 1 designed to determine magnetic intensity by electric meth- 

 ods. 2 Meanwhile the numerous comparisons with magnetic observatory stand- 

 ards indicate that these provisional standards approach sufficiently close to 

 probable international ones that they may be considered as fulfilling all practical 

 requirements of a general magnetic survey of the Earth. 



Accordingly, these provisional "International Magnetic Standards," desig- 

 nated I. M.S., have been adopted for the results contained in this volume. The 

 results already published in Volumes I, II, and III were reduced to the standards, 

 designated C.I.W., adopted before the compilation of intercomparison data made 

 possible the adoption of provisional "International Magnetic Standards"; they 

 may be referred to I. M.S. by the following relations: 



Declination,!): I.M.S. = C.I.W. -O'.l 



Inclination, 7: I.M.S. = C.I.W. +0'.5 



Horizontal intensity, H : I.M.S. = C.I. W. - 0.00015# 



The instruments used as standards by the Department during 1914 to 1920 

 were the same as those used prior to 1914 for results given in Volumes I and II, 

 viz: In declination, C.I.W. magnetometer No. 3 with correction on I.M.S. of O'.l 

 to observed values; in horizontal intensity, C.I.W. magnetometer No. 3 with zero 

 correction on I.M.S. to observed values; in inclination, earth inductor No. 48, 

 made by Schulze, with zero correction on I.M.S. to observed values. 



MAGNETOMETER CORRECTIONS. 



The corrections of each magnetometer on the adopted standard were determined at 

 Washington, before and after use of the instrument in the field, and also, whenever 

 possible, in the field by means of intercomparisons with other outfits. The accuracy of 



1 The Schuster-Smith magnetometer, constructed at the National Physical Laboratory, and the sine galvanometer, 

 designed by Dr. S. J. Barnett and constructed by the Department of Terrestrial Magnetism, were completed early in 1921. 

 It is greatly hoped that the expectations as regards high absolute precision of intensity determinations with these instru- 

 ments may be fully realized and that early intercomparisons may be possible between them and Btandard magnetometers 

 of different countries, in order to assist in determining upon international magnetic standards. 



2 See L. A. Bauer, Terr. Mag., vol. 19, pp. 1-18, 1914; N. E. Dorsey, Terr. Mag., vol. 18, pp. 1-38, 1913; W. A. 

 Jenkins, Phil. Mag., vol. 26, pp. 752-774, 1913; E Mauz, Physic. Zs., vol. 22, pp. 11-15, 1921; A. Schuster, Terr. Mag., vol. 

 19, pp. 19-22, 1914; A. Tanakadate, Proc. R. S. Edinburg, vol. 12, 1883 to 1884, and J. Coll. Sci., Tokio, vol. 2, pp. 160- 

 262, 1888; N. Watanabe, Proc. Phys.-Math. Soc. Japan, ser. 3, vol. 2, pp. 210-223, 1920; W. Watson, Phil. Trans. R. A., 

 ser. A, vol. 198, pp. 431-462, 1902. 



