PART 111. EARTH CURRENTS AND EARTH MAGNETISM. CHAP. I. 



735 



The scale-values give the current in amperes corresponding to a deflection of i mm. on the origi- 

 nal magnetogram. 



In order to calculate J e from the copies of curves given in the plates we must further know the 

 iint-resistance used in each case, and the galvanometer used in the two directions. These data are 

 given in the following table for the various plates. 



TABLE CXI. 



The shunts will also be put up on each plate, where 



-St is the shunt in E W circuit 

 S 3 N S 



With the exception of the two-hourly records of November, the direction can be found from the 

 bllowing rules: 



For the N S curve a deflection upwards corresponds to a current from north to south, and for 

 he E W curve a deflection upwards corresponds to a current from east to west. 



The perturbing forces can be calculated from the curves in the usual way by using the scale- 

 values given in Table II, Part I, p. 50. The direction can be found from Table VIII, p. 59, or from 

 .he rule that on the plates a deflection upwards corresponds to increasing H. I., increasing westerly 

 declination and increasing numerical value of V. I. 



The sensitiveness given in the tables corresponds to the curves on the original photogram. The 

 ;ensitiveness to be employed in each case can easily be found by measuring on the base-line of the 

 ilate the length ([) which corresponds to one hour. Then we obtain for the scale-value to be employed 





/ 



24 

 / 



for 24-hourly records 



is the scale-value corresponding to the original photogram; / is to be measured in cm. 



