Electric Currents in the United Kingdom. 



105 



when the unit pole traverses the a. circuit was also calculated, 

 using the data as to the form of the earth given by Captain 

 Clarke and quoted by Prof. Everett (' C.G.S. System of 

 Units,' ed. 1891 , p. 71). The numerical values thus obtained 

 differed from those given above, but the differences between 

 them were of the same order. Both methods of calculation 

 lead to opposite conclusions as to the directions of the hypo- 

 thetical currents according as the 1886 or 1891 curves are 

 used ; thus proving that the small outstanding uncertainties 

 as to the magnetic state of the United Kingdom are the cause 

 of the discrepancies, which are not reduced by using a closer 

 approximation to the form of the earth. 



Conclusion. 



I can only conclude from these various figures that the 

 local magnetic surveys furnish no evidence of vertical elec- 

 trical currents in the United Kingdom. The largest number 

 obtained from the larger circuits is less than half that which 

 Dr. Schmidt assigns as the mean value for the whole earth. 

 Different calculations lead to results of different signs for the 

 same quantity ; and the data which would a priori be accepted 

 as the best give the smallest values. 



As far as the terrestrial curves are concerned, the final 

 results for the two surveys are embodied in the 1886 and the 

 mean 1891 curves respectively. The local irregularities in 

 the North of Ireland are so great that the calculations based 

 on the true isomagnetics in that country may be neglected 

 as compared with the probably much better results obtained 

 in Great Britain. 



Selecting, then, only the most trustworthy values, the 

 results may be summed up as follows in terms of amperes 

 per square kilometre. 





Circuit. 



a. 



/3. 



7- 



1886. 

 1891. 

 1891. 



-0-026 

 +0-001 



-0-004 

 -0 005 



-0-008 



From these figures we may conclude that there is not in 

 the United Kingdom a vertical current amounting on the 

 average to 0*1 ampere per square kilometre. They are not 



