496 Prof. E. Edlund on the. Magnitude of 



current and the heat produced by it would be created out of 

 nothing. The theory of unipolar induction which I have 

 proposed in the memoir referred to, on the other hand, is 

 perfectly consistent with the mechanical theory of heat : it is 

 based only upon known properties of electricity, and it explains 

 all the know r n phenomena of unipolar induction, qualitatively 

 as well as quantitatively. 



We arrive at the same result by the following reasoning. 

 At a given point in the magnetic field which surrounds a 

 magnet, a small freely suspended magnetized needle takes up 

 its position along the line of force which passes through the 

 point in question, and the force with which the magnet acts 

 upon the magnetized needle is proportional to the intensity of 

 the magnetic field at the same point. Let us now imagine a 

 vertical magnet and a small magnetic needle suspended near 

 it: the needle places itself along the line of force passing 

 through the point of suspension of the needle. If, now, the 

 magnet is made to rotate on its axis, we know by experiment 

 that the magnetized needle does not change its position, or, 

 in other words, that the lines of force are not changed in posi- 

 tion by the fact that the magnet has been put into rotation on 

 its axis. 



The intensity with which the magnet acts upon the poles of 

 the magnetized needle is thus independent of the rotation. 

 The rotation, then, exerts no influence either upon the positions 

 of the lines of force or upon the intensity of the magnetic 

 field. If a conductor moves in a uniform magnetic field, such, 

 for example, as that produced by the horizontal component of 

 the earth's magnetism, it is generally admitted *, and experi- 

 ment shows the justice of the conclusion, that the electro- 

 magnetic force of induction produced in the conductor is 

 determined by its velocity, by the angle between the direc- 

 tion of its motion and the lines of force, and finally by the 

 intensity of the magnetic field. The same rules will of course 

 hold good also in the case where the magnetic field is produced 

 by a magnet placed in the neighbourhood, although the inten- 

 sity of this field diminishes then with the increasing distance 

 of the magnet, and the lines of force are not parallel, as was 

 the case in the terrestrial magnetic field, but converge towards 

 the poles of the magnet. If, consequently, a conductor is in 

 movement in the magnetic field surrounding a magnet, the 

 electromotive force produced in the conductor depends upon 

 its velocity, on the intensity of the magnetic field at the points 



* See, amongst others, Blavier, Des grandeurs eleclriques. p. 411 

 f Paris, 1881). 



