24 Land Magnetic Observations, 1905-10 



Total intensity observations. For stations in high magnetic latitudes, the 



determination of total intensity, F, with the dip circle by Lloyd's method has been 



extensively used, and values of horizontal intensity, //, have been obtained by the 



formula, 



H = F cos I 



where I is dip. A complete specimen observation and reduction is shown on pages 

 38 and 39; see also p. 29. As this method is a relative one, it is essential that no 

 change be made in the weight used nor in its position and that the magnetization 

 of the two needles remain unchanged after the constants have been determined 

 at some base station. A frequent control of the constant C, which occurs in the 

 reduction formula, is essential; from the repeated determinations, the necessary 

 corrections for loss of magnetic moment of deflecting needle as well as temperature 

 correction can be applied to the observed values of F. The formula is: 



F = C v'cos /' CSC u CSC III 



where /' is the loaded dip angle, u = I I' , and U\ is the single deflection angle. 

 In the modified form of Lloyd-Creak dip circle, as already described, two deflecting 

 distances may be used and thus there result two independent determinations of the 

 total intensity. 



Inclination observations. The form of observations with the dip circle as well 

 as of computation is fully shown on pp. 36 and 39. The values of dip indirectly 

 obtained from the deflection observations, when observing for total intensity by 

 Lloyd's method, are utilized, provided that, for range of dip covered, reliable 

 corrections to standards are available. For specimen form of observations and of 

 computation with the earth inductor see p. 41. 



Determination of Constants. 



The constants required in the reduction of the magnetometer observations are 



(a) value of one division of scale used in reading position of suspended magnet; 



(b) the deflection distances; (c) measurement of dimensions and mass of auxiliary 

 inertia bar for computation of its moment of inertia; {d) the moment of inertia of 

 the long magnet together with its suspension; (e) induction coefficient of the long 

 magnet; (/) distribution coefticients P and Q; (g) temperature coefficient of the 

 long magnet. 



The measurement of the deflection distances and those for dimensions and mass 

 of the auxiliary inertia bar are made by the usual means of standard meter and 

 comparator, micrometer calipers, and balance; these are all referred to standard 

 temperature of 20 centigrade. The inertia bars generally used are cylindrical 

 and of drawn brass; occasionally, owing to non-homogeneity of the material, some 

 of the bars are found imperfect and must be rejected. The adopted moment 

 of inertia of the long magnet depends upon several well-tested inertia bars. For 

 this purpose, instruments of the same type built by the Department have their 

 parts interchangeable. 



