160 Intelligence and Miscellaneous Articles. 



ON THE ELECTEICAL AFTERCURRENTS OF TRANSVERSELY 

 MAGNETIZED IRON RODS. BY PROF. H. STREINTZ. 



The experiments referred to in this paper were carried out by the 

 author in conjunction with Dr. F. Streintz. 



The phenomenon was discovered by Villari, that in a rod of iron 

 or steel through which a galvanic current has passed, when agitated 

 after the interruption of this current, a galvanic current is again 

 generated the same in direction as that originally conducted through 

 the rod. He subsequently investigated some properties of these 

 aftercurrents, and gave an explanation of the phenomenon: — namely, 

 that the rod is transversely magnetized by the current ; and if we 

 employ the notion of molecular magnets, these arrange themselves 

 in concentric circles around the axis of the rod. Now, if the rod 

 be agitated after the interruption of the current, the molecular 

 magnets obey the direction-force which tends to mingle them again 

 irregularly. We can here avail ourselves of the representation of 

 the molecular magnets returning to the positions which they had 

 before the action of the current. Now, in this return they generate 

 in the rod an induction-current, to which Villari gives the name of 

 " agitation-current," but which may with equal suitableness be 

 designated as an aftercurrent. 



Afterwards H. Herwig studied the properties of transverse mag- 

 netism on iron tubes. 



The author now shows that in a very simple manner the quantity 

 of the magnetizing force can be calculated which is exerted upon 

 the molecular magnets by the current originally conducted through 

 the rod. Starting from Biot and Savart's theorem, that a recti- 

 lineal current of infinite length acts upon a pole of a magnet with 

 a force inversely proportional to the perpendicular distance of the 

 pole from the conductor, the problem is reduced to one of the 

 plane ; so that we have to calculate the action of a circular surface 

 with uniform mass (the rods investigated had a circular cross sec- 

 tion) upon a mass-point situated in the surface. 



In accordance with the laws of force previously stated, however, 

 a circular line with uniform mass exerts no action on a point 

 situated in the enclosed surface, while it acts on an external point in 

 the same plane as if the mass of the circle were collected in the centre. 



Now by this the calculation becomes very simple, and we obtain, 

 as the force which is exerted upon a magnetic pole situated at the 



distance r from the axis, jp=— , in which 7c is a constant, and a the 



semidiameter of the rod. The total moment upon all the molecular 

 magnets contained in the rod is then R=KZrt, where K, again, is 

 a constant, and I denotes the length of the rod. 



The author has also investigated by experiment the properties of 

 the aftercurrents ; and in so doing he found some confirmed which 

 could be foreseen from the theoretical developments, and also dis- 

 covered various other properties, some of them indeed very striking, 

 which could not have been determined a priori. — Kaiserliche Aka- 

 demie derWissenschaften in Wien,math.-nattiriu . Classe, Dec. 13, 1877. 



