PART III. KARTH CURRENTS AND EARTH MAGNKTISM. CHAP. I. 743 



i) the fact that every change occurring here will be felt comparatively powerfully in places where the 

 ays come very near to the earth. 



Kven if, as the observations seem to show, the magnetic forces, in absolute value, are only 

 omparatively small, or at any rate are more restricted in their effect as compared with the forces at 

 vork in the great perturbation-systems, these rapid changes will now be assumed to generate particularly 

 jowerful induced currents, as the strength of these currents approximately is only proportional to the 

 apidity with which the change takes place, and not to the strength of the external current. 



The apparently more rapid decrease outwards in the effect of these rays than in the other systems, 

 nay also be explained by the fact that here the rays will leave the earth in paths lying very near 

 hose by which they came in, whereas in the other systems the contrary is the case. 



It may therefore reasonably be assumed that the rapidly alternating currents observed in the earth- 

 urrent curves, accompanied by synchronous oscillations in the magnetic curves in which it is difficult 

 >r impossible to trace the influence of external forces that might be assumed to generate these currents, 

 ire mainly created by induction of the above-mentioned systems of rays which descend towards the 

 arth between the positive and negative polar systems of precipitation, and far from the place of 

 rgisierings. 



In more northerly latitudes too, there are possibly local storm-centres, which will have a power- 

 ul inductive effect. In fact the curves for Axeleen are disturbed almost at any time. 



There is moreover another most important point, namely, that the relation between the magnetic 

 ffect of an extra terrestrial system of the form we find during polar storms, and the effect of the in- 

 luced current-system, decreases with increasing distance from the inducing current-system, and t/uts 

 lie farther we get from the external current-system, the more strongly would the induced current be Jclt. 

 Ve shall prove this relation more fully later on. In other words the earth-currents are able to bring to 

 ower latitudes a message of a great many distant perturbations with their centres in the vicinity of the 

 >oles in cases where the external systems are too weak to cause any appreciable direct effect on our 

 nagnetometers. 



In this way we may understand that in lower latitudes most observers in a great number of cases 

 lave found the magnetometer variations to be such as would be produced by the earth-currents flowing 

 inderneath the magnets, and still external currents may be the primary cause of the magnetic distur- 

 )ances. 



From what has been said it will be evident that we cannot usually expect to be able to trace out 

 he cause of the earth-currents at a certain station from a comparison with magnetometer-records from 

 he same station. 



Very often the galvanometers during polar storms merely perform rapid oscillations about the normal 

 ine (see Series II, Nos. t, 3 & 14, PL XXXI and XXXII); but in some cases of somewhat small and regular 

 >olar storms with their centres in the vicinity of the stations, earth-currents were observed varying in 

 i regular way, and in accordance with the view that earth-currents are induced from changes in the 

 irimary external systems. 



The most typical instance of such a regular curve is the perturbation of the loth of February, but 

 he same type of correspondence between earth-current variations and polar storms is very well brought 

 >ut in a number of other storms. I will direct attention to a few of these. In Series I, January 13, 

 ibout i8 h 18'' 30 m , February 13, about i9 u 2o h ; in Series II, December 26, about 23'' 24'', January 24, 

 ibout i8 h , February 9, about 18''. 



In all these cases the following typical correspondence is found. 



