REDUCTIONS TO STANDARD INSTRUMENTS. 



The world-wide operations of the Department have necessitated extensive 

 intercomparisons of magnetic instruments at Washington as well as in the field 

 and at magnetic observatories in the regions covered. With the data thus obtained 

 it has been possible to refer the magnetic elements for the entire region embraced 

 in this publication to magnetic standards within an error, in general, on the order 

 of the error of observation. While the adopted standards are termed "provisional," 

 the numerous comparisons with Magnetic Observatory Standards, the general 

 results of which are given in Table No. 2, show that they approach so close to 

 international ones that the corrections here adopted may be considered as ful- 

 filling all practical requirements of a general magnetic survey of the Earth. A 

 separate monograph will be devoted to the observatory intercomparisons. It must 

 suffice at present to refer the interested reader to a preliminary communication 

 made by J. A. Fleming in Terrestrial Magnetism and Atmospheric Electricity, v. 16, 

 pp. 61-84, 137-162; for a preliminary note by L. A. Bauer on an "International 

 Magnetic Standard," see same journal, v. 12, pp. 161-164. 



In the first part of Table 2, the mean results of direct comparisons between 

 the Observatory standards and those of the Department are given ; the second part 

 contains the mean results as derived by making use of the data obtained by other 

 organizations, showing the relation of their standards to those of some of the obser- 

 vatories mentioned in the first part. Throughout the table east declination, 

 northerly dip, and horizontal intensity are regarded as positive; the differences, 

 "C. I. W. standards minus Observatory standards," accordingly, are to be taken 

 algebraically. Thus, since at Potsdam the magnetic declination is west, hence 

 negative, and the dip or inclination is plus, the interpretation of the figures given 

 in the first line of the table is as follows: West declination as observed with the 

 C. I. W. standardized instrument (Mag'r. No. 5) was, on the average, 0'.3 less than 

 that given by the Potsdam standard; the inclination observed with the standardized 

 dip circle (No. 177) used by the C. I. W. observer was, on the average, 0'.3 smaller 

 than the value obtained with the Potsdam earth inductor; the mean horizontal 

 intensity resulting from the observations with the C. I. W. standardized mag- 

 netometer (No. 5) was 0.00024ff or 0.000046 C. G. S. unit larger than that with the 

 Potsdam standard. 



Magnetic Standards Adopted. 



The standards adopted by the Department for reduction to a common basis of 

 the results contained in this volume are as follows: In declination, C. I. W. Magne- 

 tometer No. 3 without correction; in horizontal intensity, C. I. W. Magnetometer 

 No. 3 with a correction of -fO.OOOlSTJ applied to observed values of //, the hori- 

 zontal intensity; in inclination, Earth inductor No. 48, made by Schulze, with a 

 correction of 0'.5 applied to observed values of inclination, this correction being 

 based upon direct comparisons with the standard earth inductors at the Potsdam 

 and at the Cheltenham magnetic observatories. 



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