120 Prof. R. Clausius on the 



the magnetic moment of the iron core. We must distinguish 

 between this magnetic moment and the components mentioned 

 at the end of the previous paragraphs. The component P 2 , 

 which represents a magnetic moment whose axis coincides 

 with the axis of N, has no moment of rotation which acts 

 upon the rotating coil. The component P x , on the contrary, 

 which represents a magnetic moment the axis of which is at 

 right angles to the N axis, gives a moment of rotation which 

 is proportional to the magnitude P x . Accordingly the work 

 effected by this ponderomotive force in the rotation, which, like 

 the previous work, is also proportional to the number of turns, 

 and is negative for a positive rotation, may be represented by 



in which k is a fresh constant. 



By adding together the two expressions which represent 

 the separate ones for the work, we obtain an expression for 

 the whole work done by the ponderomotive force in the time 

 T, that is to say, 



T=-AMNt?-*NP 1 v (17) 



From this work of the ponderomotive force the work of 

 the electromotive force may be deduced by the aid of equa- 

 tion (11); that is to say, 



Ei=JM'Nv + JcNF 1 v- P i 2 v. . . . (18) 



If in these equations for P x we substitute its value given 

 at the conclusion of the preceding paragraph, we obtain 



T=-Mn(a + t ^),, .... (19) 

 Ei=MN(*+ j^Jv-ptV . . . (20) 



§ 9. Work of the Ponderomotive and Electromotive Force for 

 the Case in which the Iron Core shares in the Rotation of the 

 Coil. 



In previous determinations it was assumed that the iron core 

 of the rotating coil was at rest, and only the coil rotated. In 

 the machines in actual use the iron core is usually rigidly 

 connected with the coil and rotates with it. The question 

 arises whether, and in how far, a change is thereby produced 

 in the action. 



When the iron core rotates, the poles of the magnetism 

 worked in it constantly change their position in the iron itself 

 but they retain it in space.- It follows from the latter fact 

 that the individual windings of the coil, although in the com- 

 mon motion they retain their position unchanged in respect of 



