Magnetomotive Force. 209 



an iron ring whose permeability =/*. Then 



tJ»-"- 10 -» 



gives the magnetic induction in the iron. It was by mea- 

 suring this quantity in rings that Rowland determined the 

 values of /j, under different magnetic inductions (Phil. Mag. 

 xlvi. p. 140). 



Now it is possible from the above equation, supposing one 

 of Rowland's tables to be correct, to tell what the magnetic 

 induction and permeability for soft iron would be in the above 

 case. The induction is ten times the permeability ; we have 

 therefore only to find the corresponding point in Rowland's 

 table for soft iron, reduced to C.Gr.S. measure. I take the 

 liberty of transcribing the two columns required from Phil. 

 Mag. xlvi. p. 151. 53 is Rowland's Q reduced to C.Gr.S. 

 by dividing by 10. 



71-5 



600-5 

 966-7 

 2460 

 2923 

 3082 

 4959 

 5482 

 5782 

 6651 



390-7 



7473 ... 



p. 

 ... 2367 



868-7 



8943 ... 



... 2208 



1129 



10080 ... 



... 1899 



1936 



12270 ... 



... 1448 



2078 



12970 ... 



... 1269 



2124 



13630 ... 



... 1137 



2433 



14540 ... 



... 824-1 



2470 



15770 ... 



... 461-8 



2472 



16270 ... 



... 353-8 



2448 



16600 ... 



... 258-0 





17500 ... 







The pair of values most nearly corresponding to 53 = 10//, is 

 53 = 12970, /x = 1269. 



The observation that jjl as a property of the iron must be a 

 function of its condition, and probably of 53, was made by 

 Rowland. We see that it cannot possibly depend on «£) as is 

 usually supposed, as this is simply the field-intensity produced 

 in air by the given magnetomotive force, and has nothing to 

 do with the iron. 



In the present case (iron ring) 1q is the numerical ratio of 

 53 and //,, and so a function of the iron ; but, in broken cir- 

 cuits into which air-resistances enter, //,, the permeability of 

 the circuit, depends in general chiefly on the air-resistances. 

 So that, for given values of »£), the functions of the iron may 

 have widely differing values, according to the value of the 

 external resistance of the circuit. 



As another example we may suppose the current in the last 



