PART II. POLAR MAGNETIC PHENOMENA AND TERRELI.A EXPERIMENTS. CHAP. II. 



433 



along them. A number of points could then be determined by interpolation, upon the various sets of 

 curves where this relation had a constant value. It thereby became possible to draw upon a chart 

 curves in which this relation was constant. Along these "isogonic" lines ('), the lines offeree or the current- 

 lines, form equal angles with, for instance, the meridian. The tangent directions were now drawn in a 

 series of short, parallel strokes, which intersected the various isogonic lines; and by employing a suffi- 

 cient number of these, the chart could be as thickly covered with these small tangent directions as 

 might be desired. Lines of force and equipotential lines could then at once be drawn. 



With regard to the equipotential lines, care must be taken that those drawn are equidistant. 



We may here use the formula (23), or as we know that the potential along the parallel circles and 

 meridians varies respectively as 



f . 



o SI 



o sin OPi,i i 



and 



f# 

 J 9 



\\\: may either by calculation or by graphic or by numerical integration easily find out the different data, 

 necessary for this purpose. 



As regards the lines of force, it will be seen that they all point in towards the two characteristic 

 points, the points of convergence and divergence, so that here, in drawing, we have two fixed points 

 and also a distribution of tangent directions to hold to. 



We must finally not omit to remark that while we have drawn equipotential lines in such a way 

 that the magnetic intensity in a horizontal direction is in inverse ratio to the distance of the equipotential 

 lines, the distance between the lines of force gives no indication of the intensity. The reason of this is 

 that the lines of force give only the lines for the horizontal components, and not the total magnetic force. 



Field of force for a polar current-system of the assumed form. 



$ = 20, A = 75- A V = 0,2 1 8 i. 



Fig. 179. 

 (') Cf. J. W. SANDSTROM: Ober die Bewegung der Fliissigkeiten, Annalen der Hydrographie und maritimen Meteorologic, 1909, p. 242. 



