482 Intelligence and Miscellaneous Articles. 



ON THE DETERMINATION OF THE QUANTITY OF MAGNETISM OF 

 A MAGNET. BY R. BLONDLOT. 



The notion o£ instituting a method of magnetic exploration based 

 on the production of induced currents has long been entertained. 

 In 1849 Van Eees published* the result of researches on the dis- 

 tribution of magnetism, the principle of his process being as fol- 

 lows : — The wire of a much-flattened induction-coil is connected 

 with a galvanometer ; the bar to be examined is introduced into 

 the Coil up to a fixed point of the latter, and is then briskly with- 

 drawn to a great distance : this gives rise to an induction-current, 

 which deflects the needle of the galvanometer a certain angle. 



Van Eees lays down a simple proportionality between the inten- 

 sity of the current and the inducing magnetism, from which it fol- 

 lows that the current observed is a measure for the sum of the free 

 magnetism over which the coil glides ; and he concludes, from a 

 known relation, that that sum is equal to the actual magnetism at 

 the place from which the withdrawal of the coil started. 



Subsequently, in 1861, in a memoir an abstract of which appeared 

 in Poggendorffl's Annalenf, M. Eothlauf treats the same subject, 

 commencing with a critical examination of Van Eees's memoir. 

 The theory of the latter is faulty in two points; the principal 

 charge against it is that it supposes the experiments to be made 

 with a coil formed of one circumvolution only, and that the points 

 situated beneath it are the only ones which act by induction. We 

 refer for the details of the criticism to M. Eothlauf 's memoir^. 



Finally, M. Gaugain has recently taken up Yan Eees's method, 

 and made it the foundation of researches, which he is pursuing 

 with success, on magnetism. 



It appeared to us important to examine Van Eees's method from 

 the theoretical point of view, to seek the exact signification of the 

 numbers given by it, and to treat in particular a case in which, 

 though generally inaccurate, its application does not involve any 

 appreciable error. 



The first impulse measured represents, with respect to the in- 



i dt, i denoting the variable intensity 





of the current, and t the time, the limits of which are t and t v 



Let us go back to the theory of induction-currents given by 

 Neumann. 



If we have a fixed pole P, and a closed circuit B moving in rela- 

 tion to the pole, there is produced in the circuit an induction-cur- 

 rent in the inverse direction of the current which would give to the 

 circuit the motion which it actually has (Lenz's law). 



Let ds be an element of the circuit ; this element is the seat of 

 an electromotive force e ds. If the circuit B were traversed by a 



* Pogg. Ann. vol. lxxiv. p. 217. 



t " Bestimmung der magnetischen Vertheilung mittelst Magnet-Induc- 

 tion," Pogg. Ann. vol. cxvi. p. 592. 

 X See also G.Wiedemann, DieLekre von Galvanismus, vol. ii. p. 321, note. 



