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XXXIX. Notes on the Practical Application of the Theory of 

 Magnetic Disturbance by Earth- Currents. By R. T. 

 Glazebrook, D.Sc, F.R.S.* 



THE following notes contain some applications of Professor 

 Pucker's theory. 

 In fig. 1 let MBA be the line, the observatory, B the 

 power-house, P the tram, and OM perpendicular on the line 

 from 0. 



Then OM=y, MB = Z>, BP = ^, BA=«. Let OB = r 1? 

 OA = r 2 , 



Fig. 1. 







Prof. Pucker's expression for the vertical force at due to 

 an element dx is 



H I 1 "" T+5=^ 



n _dx 



j ub+xr+fW 



fA i =h/k = exterior conductivity/ interior conductivity. 



Now it will be shown that for many cases as a first approxi- 

 mation //, 2 # 2 /4 may be neglected compared with unity. 



On expanding in powers of jju and neglecting /n 2 a 2 /4 in 

 small terms, we find 





and this evaluates to 



* ^ b+ Vb 2 + y* J 



or /a 2 r( a -\-b)r 1 — br 2 1 a + b + r f\ 



2 L ~~tf g b + r, J* 



* Communicated by the Physical Society : read Dec. 14, 1900. 



