PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 151 



au( 



/i = 4(l-ie ^ -ie~'0, 

 ii = 4 (1 - e-^). 



(25) 

 (26) 



The slope of the first curve is at the outset somewhat greater than 

 that of the secondary curve, but eventually becomes less, the curves 

 intersecting at a point Y. The area between the curve and the asymp- 

 tote drawn parallel to the axis of abscissas is the same for both cases. 



If the circuits just described had in common a large closed iron core, 

 the current curves for open and closed secondary circuit would be 



TIME. 



Figure 44. 



Currents in the primary circuit of an induction coil with air core, when the 

 secondary circuit is closed (full curve) and when the secondary is open. 



much less like each other than the curves of Figure 44 are, even if the 

 core were not solid. We may illustrate this fact by some oscillograms 

 from a transformer which has a laminated core. 



Figure 45 shows two typical reverse current curves for the exciting 

 coil of the magnet Q which has 2788 turns, when the circuit of a 

 secondary coil of 1095 turns is (i>) open and iC) closed. Both curves 

 rise very rapidly at the start, and then bend suddenly, so as to become 

 almost horizontal for a time, but in the first fifth of a second the curve 

 taken when the secondary is closed attains 40 per cent of its final 

 value, and the other curve only 18 per cent ; yet the second curve 

 reaches half its height about two fifths of a second sooner than the 

 first does ; and when the secondary is open the current in the primary 



