On the Theory of Unipolar Induction. 401 



the angular displacement for light of different wave-lengths, 

 as is inevitable where quartz, mica,, or sugar is used to pro- 

 duce a rotation in one half of the field. It is preferable to 

 Jellett's, because, in the Jellett prism, not only do the two 

 images of varying intensity, which are to be compared, not 

 lie in juxtaposition to one another, but there is the ordinary 

 image lying between them interfering with the comparison. 

 Lastly, the twin-prisms now described are more easy to con- 

 struct than the divided Nicol prisms of Cornu and of Schmidt 

 and Haensch. 

 September, 1887. 



LIII. On the Theory of Unipolar Induction. By Prof. E. 

 Edlund, Professor of Physics at the Royal Academy of 

 Sciences of Sweden*. 



§1. 



INCREASE or decrease in the intensity of the pole of 

 a magnet produces, in a conductor which is approach- 

 ing it, an electromotive force tending to create an electric 

 current. The increase or decrease in the distance between 

 the pole and the conductor gives rise to a like force, seeing 

 that the action of the pole increases or diminishes in this case 

 as in the former. The electric currents proceeding from this 

 cause are said in both cases to be due to magneto- electric 

 induction ; but it is also possible to produce electric cur- 

 rents by means of a magnetic pole without increasing or de- 

 creasing the intensity of the pole, and without increasing or 

 decreasing the distance between the pole and the conductor. 

 It is said, then, that the production of the electric current is 

 due to unipolar induction ; an expression which is in no sense 

 appropriate, seeing that the conditions necessary for the pro- 

 duction of the first-mentioned currents are not applicable to 

 the production of the second, and that moreover the addi- 

 tional term unipolar is the result of a mode of explaining the 

 origin of the currents mentioned which is actually without 

 meaning. These currents are not produced by induction, in 

 the proper and original meaning of the term, but they appear 

 rather, as will be shown further on, to have a magneto-electric 

 origin — that is to say, to be due to the direct action of the 

 magnet on an electric current. It is chiefly in Germany that 

 the theory of the so-called Unipolar Induction has been de- 

 veloped, and attempts made to defend the theory against 

 objections which have been recently raised against it. 



* Translated from a memoir presented to the Academy of Sciences of 

 Sweden on the 9th of March, 1887. Communicated by the Author. 



