Comparisons of Magnetic Standards, 1915-21 397 



inclination, earth inductor No. 48 with a correction of 0'.5 applied to observed values 

 of inclination. As a result of the preliminary investigations in Volume II (pp. 271 

 273) of the results obtained during 1905 to 1914 we arrived at what were provisionally 

 termed "International Magnetic Standards" (I. M.S.), which were shown to have re- 

 mained constant within all practical requirements. These standards have been used 

 for all the data given in the present report. The results of the comparison observa- 

 tions following (pp. 397-474), have confirmed the belief that the "I.M.S." would be 

 found to answer all practical purposes. No special explanation is required except that 

 "I.M.S." means the result obtained by the observer of the Department of Terrestrial 

 Magnetism of the Carnegie Institution of Washington as reduced to the standards just 

 specified. Additional details regarding the comparisons will be found in the final sum- 

 maries (pp. 474, 475). 



Throughout the tables, declination, D, east, and inclination, /, north end of needle 

 below horizon, are designated by the plus sign. The difference, I.M.S. Observatory, 

 is taken algebraically. Horizontal intensity, H, is regarded as plus, whether the value 

 applies to the northern or to the southern magnetic hemisphere. For convenience in 

 expressing the /^-differences, the values of H are given in gammas (y), i.e., in units of 

 the fifth decimal C.G.S. The mean //-difference is, furthermore, expressed in parts of 

 the observed H for the purpose of facilitating its application to places of different H. 

 It is not correct, as may have been first pointed out by L. A. Bauer," to assume that the 

 intensity correction of a magnetometer, expressed in absolute units, will remain the same 

 with change of magnetic field, the amount of the correction depending, in fact, upon 

 the absolute value of the intensity at the place of observation. From whatever source 

 the correction generally arises, it can be expressed, with close approximation, by a 

 simple ratio change, i. e., a factor multiplied into the first power of the value of the 

 intensity; only in certain extreme cases will a second term, involving the second power 

 of the intensity, enter appreciably. 



It is a pleasure to record our indebtedness to the directors of the various observa- 

 tories, and to the members of their staffs, for the very cordial assistance rendered, as 

 well as our appreciation of the uniform courtesies extended to the representatives of 

 the Department. 



NO. I. AGINCOURT OBSERVATORY, NEAR TORONTO, CANADA. 



Comparisons at Washington, 1915. 



Comparisons of field magnetometer No. 15, of the Meteorological Service of Canada, 

 and of earth inductor Toepfer No. 89, the standard of the Agincourt Observatory, were 

 secured in November and December 1915 at the Standardizing Magnetic Observatory 

 of the Department of Terrestrial Magnetism at Washington, D. C, by Observers W. 

 E. W. Jackson of the Meteorological Service and H. W. Fisk of the Department. 

 C.I.W. standard magnetometer No. 3 and C.I.W. standard earth-inductor Schulze No. 

 48 were used in the comparisons. The Meteorological-Service magnetometer No. 15 

 is a theodolite-magnetometer made in the workshop of the Department of Terrestrial 

 Magnetism, and is similar to type 1(6) described in Volume I, Researches, Department of 

 Terrestrial Magnetism, pages 3 to 5. The observations were made at the two tripod- 

 stations N m and S m , and at the three piers N t , S e , and E m , of the Standardizing Mag- 

 netic Observatory; a full description of this Observatory and a plan showing the loca- 

 tions of the stations is given in Volume II, pages 199 to 200 and Fig. 9. The galvanom- 

 eters used for the observations with the earth inductors were mounted on piers N g , 

 S a , and E m . 



" Terr. Mag., Vol. 12, p. 161, foot-note. 



