Comparisons of Magnetic Standards, 1905-14 213 



present volume (pp. 5-15), to identify and to describe them. The magnetometers are almost 

 invariably of the design of the Department, in most cases constructed directly in its own 

 instrument shop, or according to its own specifications. The dip circles, with the exception 

 of No. IS by Casella, are all of Dover make, with certain modifications in some cases as 

 specified by the Department. The earth inductors Nos. 2 and 48 are of the Wild-Eschen- 

 hagen type, the first constructed by Toepfer, of Potsdam, and the latter by Schulze, also of 

 Potsdam. The Department is now constructing its own earth inductors.^ 



The corrections applied to the magnetometers and dip instruments in order to refer the 

 results obtained to the provisionallv adopted standards of the Department are as enum- 

 erated in Vols. I (pp. 44-50) and 11 (pp. 16-20). 



The provisional standards of the Department of Terrestrial Magnetism for the com- 

 parison results obtained up to the end of 1914 were the .same as for the results of the field 

 work during the period 1905-1913, namely: For declination, C. I. W. magnetometer No. 3 

 without correction; for horizontal intensity, C. I. W. magnetometer No. 3 with a correction 

 of +0.00015// applied to observed values of //, the horizontal intensity; for inclination, 

 earth inductor Yo. 48 with a correction of 0.'5 applied to observed values of inchnation. 

 The possible errors of the provisional standards are discus.sed later (pp. 271-273). 



The tables of comparison-results for the various magnetic observatories, or services, 

 will be found given in alphabetical order. No special explanation is required, except that 

 C. I. W., standing for "Carnegie Institution of Washington," means the result obtained 

 by the observer of the Department of Terrestrial Magnetism of the Carnegie Institution 

 of Washington and reduced to the provisional standards as just specified. The reductions 

 to "International Magnetic Standards" (I. M. S.) are made in accordance with the state- 

 ment on page 273. Additional details regarding the comparisons will be found in the 

 final summaries (p. 278). 



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

 below horizon, are designated b}' the plus sign. The difference, C. I. W. Observatory, 

 is taken algebraically. Horizontal intensity, H, is of course to be 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 (7), 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 one of the writers,- 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 general^ 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 in- 

 tensity, enter appreciably. 



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



Three stations were used in the 1906 comparisons at the Agincourt Observatory: for 

 declination a tent station, T, was chosen as close as possible to the declination pier, which 

 could not be occupied without dismounting the Observatory instrument; for horizontal 

 intensity both T and the regidar intensity pier, /, were used; for the dip work, T, I, and the 

 regular dip pier, D, were occupied. 



The standard instruments of the Observatory were for September and October 1906: 

 Toronto dechnometer for declination; Elliott magnetometer No. 48 for horizontal intensity; 



See " Description of the C. I. W. Combined Magnetometer and Earth Inductor," by J. A. Fleming and J. A. Widmer, 

 Terr. Mag., vol. 18, 1913, pp. 105-110; also this volume, pp. 9-12. 

 'L. A. Bauer, Terr. Mag., vol. 12, 1907, p. 161, footnote. 



