PEIRCE. 



BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 



161 



second (B) 250 turns, and the third (C), which is made of wire of very 

 large cross- section, has a small unknown number. Figure 56 reproduces 

 accurately the records of two oscillographs, one in the coil A, the other 

 in B, when C was closed. 03IQL is a part of the building-up curve 

 for the main circuit {A), and Ocblc is a corresponding portion of the 

 record of the induced current in B. In the case represented by the 

 full line OMQTVW, the coil C was suddenly opened at about 1.05 

 seconds after the start : Ocbznda shows the record of the induced 

 current in B under these circumstances. The scales of the two oscillo- 

 graphs were, of course, not the same. The sudden jumps in the 

 oscillograms might have been predicted, in amount as well as in direc- 

 tion, by the principle of the " Conservation of Electromagnetic Mo- 



. Y U 



SECONDS. 



Figure 55. 



menta." We shall return to the subject of the sudden changes brought 

 about in the currents in inductively connected circuits when the 

 inductances of the system are impulsively changed. 



The Effectiveness of Fine Subdivision in the Core of an Electro- 

 magnet for the Prevention of Electromagnetic Disturbances 

 due to Eddy Currents, when a Steady Electromotive Force is 

 applied to the Circuit of the Exciting Coil. 



In order to determine approximately the magnitude of the effect of 

 eddy currents upon the growth of a current 13 in the coil of an electro- 

 magnet the core of which is made of fine iron wire, we may consider 

 the case of a very long solenoid consisting of N turns of wire per cen- 

 timeter of its length, wound closely about a long prism of square cross- 



13 The influence of eddy currents in the formation of a regularly fluctuating 

 current in the exciting coil of a transformer under a given, alternating electro- 

 motive force has been studied by J. J. Thomson for cores of square cross-suc- 

 tion built up of iron sheets, and by Heaviside for round cylindrical cores cut 

 radially. See the Electrician for April, 1892, and Heaviside's Electrical Papers, 

 1, xxviii. 



vol. xliii. — 11 



