46 Dr. A. Stoletow on the Magnetizing -Function of 



induced electromotive force, expressed in absolute electromag- 

 netic measure, is, according to the theory given by Kirchhoff: — 



E=Wi{47r*M + 'Pj (1) 



The first term of the expression proceeds from the currents in- 

 duced by the remagnetizing of the iron ; the second, from the 

 direct voltaic induction of the two wires. Herein signify : — 

 n and n 1 , the numbers of the windings of the primary and 

 secondary wires respectively (if the latter is wound round 

 the ring v times in one direction, v 7 times in the other, we 

 have to understand by n' the difference v— V); 

 i, the intensity of the primary current, in absolute electromag- 

 netic measure ; 

 M, the integral, extended to the cross section of the ring, of 



the form I — , in which d$ is an element of the surface of 



y p 



that section, p the distance of this element from the rotation- 

 axis of the ring ; 

 P, a similar integral, referred to the surface of a primary 

 winding. 



k is the magnetizing-function of the iron; and the argument 

 H, to which k is referred, is the mean value of the magnetizing- 



force. This is = — for a point (p) of the ring. Consequently 



B-*j£ (2) 



.S denoting the entire surface of the cross section of the iron. 



If, then, we know the form and dimensions of the ring and 

 the primary turns, as well as the number of these and the se- 

 condary rounds, the function k can be calculated for every given 



R, to be expressed in absolute measure, as soon as the ratio — 



i 

 is likewise measured in absolute measure. 



This is the fundamental idea of the method recommended by 

 Kirchhoff and which I have followed. To what extent it required 

 to be modified for different values of the magnetizing-force R, 

 will be gathered from the following. 



I had such a ring made of soft iron, in the workshop of Dr. 

 Meyerstein, at Gottingen. It was kept for twelve hours at a 

 red heat, and then cooled by gradually covering the fire. The 

 cross section of the ring is a rectangle ; the extreme diameter I 

 found equal to 200*025 millims., the internal = 180*37 millims.; 

 the height =14*75 millims. From this the quantity denoted 

 above by M is calculated =1*526 millim. 



To this ring two rings of wood, circularly rounded off on the 



