ON THE MANNER OF GROWTH OF A CURRENT IN THE 

 COIL OF A NEARLY-CLOSED ELECTROMAGNET AS 

 INFLUENCED BY THE WIDTH OF THE AIR GAP. 



By B. O. Peirce. 



Presented December 13, 1905. Received December 27, 1905. 



In the course of a set of experiments on the time-lag in the magnetiza- 

 tion of iron, which are being carried on in the Jefferson Laboratory, it 

 has been found desirable to use, in series with the testing apparatus, one 

 or more large electromagnets to increase the inductance of the whole 

 circuit so greatly that the effects of sudden small changes in the induc- 

 tance of the testing coils may be of slight importance ; and it has been 

 necessary to study specially some of the properties of these particular 

 magnets, since, in their cases, certain of the conditions which underlie 

 the convenient methods usually employed in treating practical problems 

 are not very exactly fulfilled. In order to be able to predict the behavior 

 of a nearly-closed magnet under the conditions of the work, one needed 

 to know the effective inductance of the magnet under given conditions, 

 and the raauner of growth of the current in the coil circuit as dependent 

 upon the applied electromotive force, the final value of the current, and 

 the width of the air gap. These matters, with some others, were studied, 

 aud, apropos of the extremely interesting experiments recorded in Dr. 

 Thornton's recent article 1 on "The Magnetization of Iron in Bulk," aud 

 of the work 2 of Messrs. J. and B. Hopkinson and E. Wilson on "The 

 Propagation of Magnetization of Iron as Affected by the Electric Currents 

 in the Iron," I propose to give briefly in this paper the results of tests 

 made on one of the electromagnets I have used, as illustrating some char- 

 acteristics of nearly-closed, massive iron cores. With these results I 

 wish to compare some others obtained from a large closed electromagnet 

 with finely laminated core. 



1 The Philosophical Magazine, 8, 1904. 



2 The Philosophical Transactions, 186, 1895. Proceedings of the London Insti- 

 tute of Electrical Engineers, 1895. 



