PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGMET. 127 



coil, the current was "direct," and its rise was represented by the upper 

 curve. If the new current had a direction opposite to that of the last 

 current through the coil, the current was "reverse," and followed the 

 lower curve. The areas V and W are practically equal. 



It is evident that, other things being equal, the rapidity of rise of 

 the current in a circuit which contains a coil wound around the core of 

 an electromagnet will depend very much upon the number of turns in 

 the coil. Figure 20 shows reverse curves from the magnet R. The 

 actual strengths of the currents were 6, 3, and 1.5 amperes respectively, 

 and the numbers of turns in the exciting coils were 85, 170, and 340. 



Figure 20. 



Curves showing the growth of currents in coils of 340 turns, 170 turns, and 85 

 turns belonging to the magnet It. The same electromotive force was used for 

 all the cases, and the final values of the currents were 6 amperes, 3 amperes, and 

 1.5 amperes. 



The electromotive force was the same in all three cases. The horizon- 

 tal units are tenths of seconds. 



Although the typical current curve for the coil of an electromagnet 

 wound in many turns about the core has two points of inflexion if the 

 core is laminated, both of these disappear if the change of the magnetic 

 flux through the circuit due to the current is small enough, and 

 occasionally one finds an oscillogram which seems to have only one 

 point of inflexion. Some of the direct curves shown in Figures 5, 23, 

 and 28 are everywhere convex upward. Among the nearly three 

 thousand photographed oscillograph records taken for use in this paper 

 no one is concave upward at the very start, but a curve of this kind, with 

 one point of inflexion, has been shown by Dr. Thornton, and I have 



