PEIRCE. — CHANGES IN INDUCTANCES OF ELECTRIC CIRCUITS. 585 



eddy currents already existed changed the amount of the final flux 

 through the circuit, Mr. Coulson has measured with great care a num- 

 ber of oscillograms taken with this apparatus, and finds the area 

 between the asymptote and the curve 0AM to be 6216 on the scale of 

 his planimeter, while the area above the curve OBN is 6214 on the 

 same scale. The areas above the curves agree within a small fraction 

 of one per cent, as they were expected to do. 



Figures 40 and 41 exhibit oscillograms taken with the toroid, DN, 

 under sudden opening and closing of the second coil, and these show 

 no signs of von Waltenhofen effects. Figure 42 gives the records of 

 two oscillographs, one in the primary circuit of a toroid which has a 

 core made of soft iron wire only one tenth of a millimeter in diameter, 

 the other in a secondary coil, when a third coil, wound on the same 

 core, was suddenly closed. 



In early experiments upon the phenomenon of the reversal of mo- 

 ment in short rods magnetized in a solenoid, when the current was 

 suddenly stopped, it was observed that if the rod had been previously 

 magnetized permanently in the direction in which the current magnet- 

 ized it, reversal never occurred, but that it always appeared, under 

 favorable circumstances, if the direction of the previous magnetization 

 was the opposite of that which the current gave it. This and like 

 results has led many physicists to think that the molecules of the iron, 

 when the exciting force due to the current is suddenly removed, return 

 to the positions which they had just before the current acted upon 

 them, but that the motion is so much resisted by frictional forces that 

 the kinetic energy is lost when the particles have swung slightly be- 

 yond the positions of equilibrium where they are held by the friction. 

 Wiedemann believed, on the other hand, that when the exciting circuit 

 of an electromagnet is suddenly opened, the rise and decay of the 

 OefFnungsextrastrom induces in the mass of the iron, currents, alternat- 

 ing in direction and decreasing in intensity, and that the magnetization 

 of a rod due to the original current is reversed in sign, under favorable 

 circumstances, by a weaker current in the opposite direction. In the 

 case of closed rings, where demagnetizing factors are absent, anomalous 

 magnetization seems to appear only when eddy currents in the iron so 

 shield the particles inside the mass that they are never exposed to 

 sudden changes in the intensity of the exciting magnetic field. 



My thanks are due to the Trustees of the Bache Fund of the Na- 

 tional Academy of Sciences, who have lent me some of the apparatus 

 used in measuring the oscillograms mentioned in this paper. 



The Jefferson Physical Laboratory, 

 Cambridge, Mass. 



