104 PROCEEDINGS OF THE AMERICAN ACADEMY. 



but when large iron frames were completely closed, it became the 

 custom, in satisfying commercial contracts, to attempt to get informa- 

 tion about the permeability of the metal as a whole from tests, under 

 given conditions, upon small, thin specimen pieces made as nearly as 

 possible of the same material as the original, or else cut from it. It 

 was usually impossible, however, to be sure that the temper of the 

 small piece was sufficiently like that of the mass to make it a fair 

 representative of the whole, and the preparation of the specimens was 

 often troublesome, so that some more practical method of procedure 

 was seen to be desirable,^ and it seems to have occurred to a number 

 of different persons independently that a good deal might be learned 

 about the magnetic properties of the core of an electromagnet if 

 one determined the manner of growth of a current in an exciting 

 coil of a given number of turns wound closely about the core, when, 

 under given initial conditions, a constant, known, electromotive force 

 ■was applied to the coil circuit. 



The Determination of some of the Magnetic Properties op 

 THE Core of an Electromagnet from the March of a 

 Current in the Exciting Coil. 



If, at any instant, the total flux of magnetic induction through the 

 n turns of the exciting coil of an electromagnet is N (maxwells), if r 

 is the resistance of the coil circuit (in ohms), i the current in it (in 

 amperes), and E the applied electromotive force (in volts), then 



or — - = 10* • 7 



at 



■(?--> (6) 



and if the final value {E/r) of the current be denoted by ^^ and the 

 change in N during the time interval ti to t^ by iVi.2, 



N^,, = r■W■ fll^-i)dt. (7) 



If, now, i be plotted against the time in a curve s (Figure 3) in 

 which / centimeters parallel to the axis of abscissas represent one 

 second, and an ordinate m centimeters long one ampere, the curve 



6 Drysdale, Jour. Inst. Elec. Engineers, 31, 1901. 



