128 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



many curves which become concave upward very near the origin. In 

 current curves belonging to the coil of an electromagnet which has a large 

 closed, solid core, there are often two points of inflexion, but many of 

 even the reverse curves are everywhere convex upward. Figure 21 

 shows curves taken for the coil of the large magnet P in the circuit of 

 which was a storage battery of voltage 84. When each current started, 

 the core was nearly neutral. 



Figure 21. 



Curves showing the manner of growth of currents of various final strengths 

 in the coil of 282.3 turns belonging to the magnet P. The gap was closed and 

 the core was nearly neutral at the beginning of each current. The applied vol- 

 tage was the same (84) for all the curves. 



When the coil of a transformer, the core of which is built up of such 

 thin plates of soft iron as are used in the best practice, is subjected to 

 an alternating electromotive force of extremely high frequency, the 

 disturbing effect of eddy currents in the iron are, of course, very ap- 

 parent, but the manner of growth of a current under a constant electro- 

 motive force is usually not very greatly affected by such currents. 



The fact that the susceptibility of the iron is by no means constant, 

 materially alters the shape of a current curve when iron is introduced 

 into a circuit ; nevertheless, it is instructive to compare the manner of 



