^ 



Comparisons of Magnetic Standards, 1905-14 273 



over +()'.2 or -(-0'.3. The correction for tlie field results will not be the full amount, 

 +0'.5, for the reason that the dip-circle corrections have been based frequently, not simply 

 upon the C. I. W. standard, but also on observatory standards in various parts of the Earth, 

 in order to eliminate, as far as practicable, the error arising from possible variations of the 

 dip-needle correction with change in magnetic latitude. It was, however, found worth 

 while to correct the results of tlie observatory comparisons, as they are, on the average, 

 considerably improved thereby. 



To recapitulate: 



In order to refer the results of the observatory co}7}parisons, as given in the various tables 

 on pp. 21J^267, to international magnetic standards {I. M.S.) as accuratetij as they can be fixed 

 from the data at hand up to 1915, the following corrections have been applied algebraically to the 

 C. I. W. values: for declination, O'.l; for horizontal intensity, O.OOOloH; and for inclina- 

 tion, +0'.5. 



REGARDING THE CONSTANCY OF MAGNETIC STANDARDS. 



A word with regard to the constancy of magnetic standards. This deiaends primarilj' 

 upon the care and attention that can be paid to the instruments selected, and this in turn 

 depends not only upon the observer-in-charge, but, in no small measure, upon the construc- 

 tion and style of the instruments and the climatic conditions at the place of observation. 

 Thus, for example, if the magnetometer is one in which the magnets are not incased in some 

 protecting sheath, so that it is possible for the fingers or the moist air to come in direct 

 contact with the steel, it is quite possible, by oxidation, to have, in the course of time, 

 sufficient loss of mass to alter appreciably the moment of inertia of the principal magnet. 

 This experience has actually been encountered at magnetic observatories situated in moist 

 tropical regions. For similar reasons in such regions there is a rapid deterioration in the 

 pivots of dip needles, tiny rust spots and pits quickly developing which cause notable errors 

 in the values of the inclination, ^^^ule it is probable that earth inductors will, in general, 

 be found to be better observatory instruments than are dip circles, they also require 

 scrupulous care, if the highest possible accuracy with them is to be maintained for a 

 period of years. 



^^'ith respect to the distribution coefficients P and Q, or P' , our experience has been 

 that if careful attention is bestowed on the magnets, if they are not allowed to touch each 

 other or other masses of iron or steel, and are not subjected to violent shocks such as are 

 experienced in a fall, then it is inad\'isable to make such frequent changes in their adopted 

 values as is the practice at some magnetic observatories. .Artificial variations and serious 

 discontinuities in observatory series have, at times, been the result of such changes. 

 Indeed, some observers still tliink that it is possible to obtain good values of these coefficients 

 from a veiy limited series of observations. 



Even in some of our most strenuous field campaigns extending over periods as long as 

 2 years and more, diu'ing which the instruments were subjected to conditions not experi- 

 enced at observatories, we have found no cause to make such large changes in the adopted 

 distribution coefficients as are made occasionally at magnetic observatories every G or 12 

 months, in spite of the fact that theii' instruments have never been subjected to travel. Such 

 frequent changes are likewise very disconcerting if one wishes to test the constancj' of com- 

 parison-differences, by repeating the observations at intervals of 2 or 3 years. 



Judging from oin- experience during the 10 years, 1905-14, it seems possible, when 

 the above-mentioned precautions are adequately observed, to preserve the constancy of 

 magnetic standards without change of constants, for a period of 5 to 10 years, within an 

 accuracy of 0'.2, or less, in dechnation and inclination, and 0.00015i/, or less, in horizontal 

 intensity, i. e., about 87 or less for the average European observatory. 



