PART III. EARTH CURRENTS AND EARTH MAGNETISM. CHAP. I. 733 



It is beyond our power to give the complete series of records of the earth-currents during this period ; 

 jut we shall attempt to give, as far as possible, a true representation of the typical cases of earth- 

 .-urrent phenomena by selecting a number of disturbed days for which we have successful records. 



We are of course aware that a complete representation would have been preferable ; but such a 

 Drocedure in our case is excluded from the very fact that owing to difficulties with the galvanometers 

 iiiccessful earth-current registerings are wanting during considerable intervals, and unfortunately records 

 >{ earth-currents are wanting for a number of the very greatest disturbances. Being unable to give a 

 omplete representation, I think our procedure will be the best one, because very little would be gained 

 y giving curves for intervals during which nothing of particular interest has happened. 



The curves treated will be represented at the end of this volume in a series of plates giving a 

 lirect reproduction of the curves recorded photographically. In addition to the earth-current curves, the 

 nagnetometer registerings will be given for the same interval. The curves were copied partly photo- 

 graphically, partly by drawing on transparent paper directly from the photograms. 



On each of the earth-current curves an arrow is drawn giving the direction of the galvanic current 

 vhich produces a deflection in the direction of the arrow. 



The plates are divided into three series. 



The first series contains, in chronological order, a number of 24-hourly records representing mode- 

 ate variations. 



The second scries contains 24-hourly records of a number of comparatively great storms, in fact 

 he series contains all the great storms for which earth-currents have been successfully recorded. 



Thr third series contains a number of two-hourly records. 



Although we are unable to find absolute values of the earth-currents, it may still be of interest to 

 hid relative numbers for the current-changes which accompany the magnetic variations In this way 

 ve may for instance be able to form vector diagrams for the currents, and compare them with the 

 orresponding ones for the magnetic elements. 



The determination of the somewhat rapid changes of earth-currents only lasting for a few hours 

 an be done in a similar way as for the determination of the perturbing force, by placing on the photo- 

 ;ram a normal line harmoniously connecting the quiet parts of the curve. 

 The change of current Jim the cable is given by the equation 



G + s 

 Jl = - - Jn 



s 



is the shunt-resistance, G is the galvanometer-resistance, J n is the deflection measured on the photo- 

 ;ram, e is the scale-value for the photogram, and gives the current through the galvanometer coil which 

 orresponds to a deflection of i mm. 



The corresponding electromotive force J e between the cable terminals will be approximately 



jc = (Q + s) Jl, 



,-here Q is the resistance of the cable, and is equal to 1.55.1?, as throughout s is small compared with 



/, or with sufficient accuracy 



/~* 



4e (p -H .s) . Jn. 



s 



The quantity J r is probably not equal to the electromotive force J E between the same points in 

 ase the cable was removed. We may put 



Birkelarul. The Norwegian Eurora Polaris Expedition, 1902 1903. 



