CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



THE INFLUENCE OF THE MAGNETIC CHARACTERISTICS 



OF THE IRON CORE OF AN INDUCTION COIL UPON 



THE MANNER OF ESTABLISHMENT OF A STEADY 



CURRENT IN THE PRIMARY CIRCUIT. 



By B. Osgood PEmcE.f 

 Presented January 13, 1915. Received Dec. 1, 1914. 



Introduction. — Nearly sixty years ago, Helmholtz proved experi- 

 mentally that the predictions of the mathematical theory which had 

 been constructed to explain the courses of currents in inductively 

 connected circuits, were fulfilled when the inductances were fixed 

 constants, and shortly thereafter Du Bois Reymond put the equa- 

 tions for the currents in the two circuits of an induction coil with air 

 core, into the forms in which they appear in modern textbooks. This 

 simple theory, however, was soon found to be inapplicable when iron 

 cores were used, and we now know that we cannot predict the march 

 under given applied electromotive forces, of the currents in a number 

 of coils wound on an iron core, without a knowledge of the magnetic 

 history of the iron as well as of its magnetic properties, and that a 

 general mathematical theory — if it were possible to form one — 

 would be very complex. Notwithstanding this, it is often necessary 

 to study the magnetic characteristics of a piece of iron by making it 

 the core of a simple induction coil or transformer, and then determin- 

 ing experimentally the effect of its presence upon the manner of 

 growth of the current in each circuit when a given electromotive force 

 is applied to the primary. When the mass of the core is large, it is 

 often important that the observer shall have at the outset a general 

 knowledge of the manner in which the currents will change under a 

 given set of initial conditions; this paper discusses briefly a few typical 

 cases. 



Magnets Employed in the Experiments. — In some of the ex- 

 periments recorded in this paper, I have made use of an electromagnet 

 (J, Fig. 1), the core of which, built up of varnished sheets of iron 0.38 

 millimeters thick, has a cross section of upwards of 150 square centi- 



t Deceased. 



