242 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



constructed. The induction is assumed to be observed experimentally 

 by the step method, and the K of the table is used in the equation 



H=H' -KB. 



TABLE XVIII. 



Problem. 



Suppose the magnetic susceptibility in a soft iron rod similar to 

 Bessemer steel is to be tested ballistically. Suppose the rod is neither 

 very thick nor long, and the ballistic galvanometer (Thomson) is not very 

 sensitive. In order to get the greatest possible throw we may wind a 

 large number of turns of wire of secondary coil around the middle of 

 the rod, being careful not to exceed the point of maximum sensitive- 

 ness. This is reached when an additional turn of wire adds propor- 

 tionately more resistance to that already in the galvanometer circuit 

 than it adds turns to the total number of turns. Of course as long as 

 the secondary coil is wound on in a single layer, and the resistance of 

 the galvanometer is not negligible, this condition can never be reached ; 

 but where the coil is built up in several layers the resistance finally 

 predominates. Suppose we have : 



Galvanometer resistance = 12 ohms. 



Sensitiveness = 0.0695 mm. throw per microcoulomb. 



