440 Dr. Glazebrook on the Practical Application of the 



in a mile of line by the number trj /* 2 /8. Let the result be 

 J amperes. Then the maximum value of a is given by 



2-5J a - 1 , =1. 



V N/(a 2 + 4y 2 ) 



According to the figures assumed above the value of fi 2 /8 

 is -05. 



Again, we have 



Hence X = kv, 



as is obvious from the definitions. 



The value of 1/k is, as we have seen, '0135 ohm ; and since 

 fM 2 — h/k, we have 



y = -yr ='034 ohm approximately. 



Of course if a lower value be assumed for /* 2 the value of 

 1/h is proportionately raised. 



Again, for rails of similar cross- section h varies as the 

 linear dimensions of the cross-section, and k as the square of 

 the same. Hence jju 2 varies inversely as the linear dimensions, 

 and the leak is reduced for a given current by increasing the 

 area of ih^ rail. 



Appendix added January 1901, 



An important paper was read before the German Physical 

 Society in Berlin in June 1899 by J. Edler ( Verhandl. cles 

 Deutschen physikal. Gesellsch. i. Jahrg. No. 10), describing a 

 series of experiments in which the magnetic disturbances due 

 to a line at Spandau were measured at a number of stations at 

 different distances from the line ; and it occurred to me to apply 

 the above theory to the results. The line is approximately 

 straight, and is about 5 kilometres in length. The current 

 is said to have varied between 35 and 150 amperes. I have 

 assumed in the calculations a mean current of 100 amperes, 

 the voltage being 500 volts. I also assumed the value of the 

 leak-coefficient for a mile of line to be '025, corresponding to 

 the lesser of the two values for which tables are calculated in 

 the paper. The value of fi 2 which corresponds to this leak 

 is, when the distances are measured in kilometres, approxi- 

 mately equal to "08, and the leak in a length of a kilometres 

 is 'Ola 2 . The results for the vertical disturbance are given in 

 Table III., in which the first column gives the distance of the 

 observing-station from the line in kilometres, the second 



