CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY, 



HARVARD UNIVERSITY. 



ON THE DETERMINATION OF THE MAGNETIC BEHAVIOR 

 OF THE FINELY DIVIDED CORE OF AN ELECTRO- 

 MAGNET WHILE A STEADY CURRENT IS BEING 

 ESTABLISHED IN THE EXCITING COIL. 



By B. Osgood Peirce. 



Presented December 12, 190G. Received June 22, 1907. 



More than fifty years ago Helmholtz established, on theoretical 

 grounds, the now familiar equations for the manner of growth of a 

 current in a circuit of constant inductance under a given electromotive 

 force, and proved by a brilliant series of experiments 1 that the 

 predictions of this theory were fulfilled in practice. It appeared, 

 in particular, that if a circuit of resistance r containing a constant 

 electromotive force, E, were closed at the origin of time, the current, 

 I, would be given by the expression 



E rt 



fd-e"^), (1) 



if L were the "potential of the circuit upon itself," that is, the self- 

 inductance. The " induced current " (/) would satisfy the equation 



. L dl E " 



e = - . — = — eA (2) 



r at r 



If, therefore, / were plotted against the time, the resulting curve 

 (OGQKC, Figure 1) would have as asymptote the straight line iZG) 

 parallel to the t axis at a distance E/r above it; the current in 

 the circuit at any time (OP) would be given by the corresponding 



h 



1 F. E. Neumann, Abh. d. Berl. Akad. 1845 and 1847; Helmholtz, Die Erhalt- 

 ung der Kraft, 1847; Pogg. Ann., 83, 1851; 91, 1854; Phil. Mag., 42, 1871. 



