Comparisons of Magnetic Standards, 1915-21 403 



numerous instruments used show, almost without exception, for each one a surprising 

 degree of precision. It has been found that, while there are naturally large accidental 

 errors at times in the field determinations of distribution coefficients, the elimination 

 of such accidental errors with the accumulation of data indicates for magnetometers 

 of the types used by the Department (see pp. 6-11) practical constancy. Thus for 

 magnetometers No. 12 to 28, the magnets of all of which are of same size and style, the 

 greater the number of determinations for the coefficient P' the more nearly the value 

 approaches the mean for all the instruments. It has been found on the other hand 

 that for those magnetometers the magnets of which are sheathed in brass changes of 

 inertia must be expected in the course of field work. 



The method of packing the long magnet is the same for all of the instruments having 

 brass-sheathed magnets; the magnet, when not in use, fits snugly in a felt-lined cylinder. 

 When in field service over any long period the change in inertia for these magnets is 

 found to be practically linear with time. Investigations of such inertia changes have 

 been made for each instrument. As typical examples of such work specimen compila- 

 tions regarding C. I.W. magnetometers No. 19, 25, and 26 are given below. 



C.I. W. magnetometer No. 19 has been used extensively and under severe transporta- 

 tion conditions for several trips in South America during 1912 to 1916. 



The value of log t^K, K being the moment of inertia about the axis of suspension 

 for magnet 19L and its stirrup, determined February 28, 29, 1916, indicated a decrease 

 of 0.00121 since the determination of June 18, 19, 1912, with the same inertia-bar. 

 The equivalent change in the value of the moment of inertia for magnet 19L and its 

 stirrup is 0.18 C. G. S. unit. 



There appear to be three possible causes for such a change: (a) Loss of mass be- 

 cause of oxidation and because of constant rubbing and handling with consequent wear- 

 ing of the magnet-sheath and stirrup in the course of field work; (b) a slight displace- 

 ment of the friction-tight magnet in its brass sheath, possibly caused by frequent severe 

 bumping and jolting of the instrument-case in transit, e.g., during numerous passages 

 through rapids in the South American campaign of 1915, and (c) a shortening of the 

 sheath possibly caused by impact during transportation. 



Assuming the loss of mass according to the hypothesis (a) to be uniform in char- 

 acter the total change in the moment of inertia would be accounted for by a loss of about 

 0.08 gram. Such a loss may be reasonably probable in the course of several years' field 

 use and might be expected to take place as a function of the time during which the instru- 

 ment was in service. On the hypothesis (6) the change in the moment of inertia would 

 require a displacement of the magnet of 1.3 millimeters. Such a displacement would 

 affect materially the balance of the magnet when suspended; that is not the case and 

 the hypothesis (6) is therefore rejected. A shortening of the magnet sheath amounting 

 to 0.05 millimeter would cause a decrease in the moment of inertia of 0.06 C. G. S. unit. 

 It is not probable that there could be a shortening of the sheath, due to crushing by 

 impact, greater than 0.05 millimeter. It appears, therefore, that the change in the 

 moment of inertia of magnet 19L and its stirrup may arise from a combination of the 

 causes (a) and (c). 



The effect of the decrease of 0.00121 in the value of log t 2 K is such that values 

 of the horizontal intensity, H, computed using the value of log t 2 K first determined would 

 be too great by 0.00139i/. The difference indicated from the comparisons with the 

 standard instrument at Washington in May 1912 and September 1915 is 0.00149#, 

 which agrees substantially with the difference indicated by the observations for moment 

 of inertia. Assuming that the change in the moment of inertia took place linearly 

 during the period of field service, namely, between September 9, 1912 (1912.69), and 

 September 24 to 28, 1915 (1915.73), the annual rate of change would be equivalent 



