48 Prof. R. Clausius on the 



a cylinder which rotates with the rotating coil ; on this press 

 two contact-springs or brushes, which form the beginning and 

 the end of that part of the conductor which comprises the coil 

 of the fixed electromagnet and the outer circuit. By means 

 of the brushes the rotating coil is divided in each position into 

 two halves which form two branches of the whole circuit ; so 

 that the electric current, which passes through the rotating 

 coil from one brush to the other, divides into two halves, which 

 bifurcate from the one brush and unite again at the other. 



The iron core of the rotating coil is magnetized in a twofold 

 manner. On the one hand it forms between the poles of the 

 fixed electromagnet an armature which does not quite touch 

 the poles, though it comes very near; and by the action of the 

 fixed electromagnet it is so magnetized that to each pole of 

 the fixed electromagnet the opposite pole of the iron core is 

 presented. In order briefly to express the latter, we may say 

 that the axis of the magnetism produced in the iron core is in 

 the opposite direction to the axis of the fixed electromagnet. 

 On the other hand, the iron core is magnetized by the electric 

 current which flows from one brush to the other. The brushes 

 are so arranged that the axis of the magnetism produced by 

 this force is approximately at right angles to the axis of that 

 which is produced by the first magnetism. A magnetism 

 results from the combination of those two actions, the axis of 

 which is oblique in reference to the directions just mentioned. 

 If the iron core rotates, its poles retain a fixed position in 

 space, while in the iron core they continually change their 

 direction. 



The inductive action which takes place in the rotating coil 

 during the rotation is of a threefold character. In the first 

 place, the fixed electromagnet acts inductively upon the rota- 

 ting coil. In the second place, the magnetized iron core of 

 the rotating coil, whose poles retain their position in space as 

 long as the coil rotates, exerts an inductive action upon the 

 latter. And, lastly, the current in the coil, which changes its 

 direction in those parts which pass by the brushes, exerts at 

 any rate a partially inductive influence on the coil. 



§ 2. Law of Induction. 



In order to establish the law of induction which we shall 

 apply in the sequel, we must consider somewhat more closely 

 the currents and conductors with which we are dealing. 



The rotating coil, and that of the fixed electromagnet, con- 

 sist of several windings; and these windings are so close that 

 it makes little difference for each individual winding, and for 

 each group of windings, if we suppose that their end, instead 



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