The 

 LONDON, EDINBURGH a.vd DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



APRIL 1851. 



XXXVI. On the Laws of Magnetism. 

 By John Tyxdall, Ph.D., Marlurg"^. 



§1- 



THE subject of the present memoir is embraced by the fol- 

 lowing four propositions : — 



1 . To determine the general relation betvjeen the strength of an 

 electro-magnet and the mutual attraction of the magnet and a mass 

 of soft iron, when both are in contact. 



2. To detonnine the said relation when the magnet and the mass 

 of soft iron are separated by a fixed distance. 



3. A constant force being applied to the mass of soft iron in a 

 direction opposed to the pull of the magnet, to determine the con- 

 ditions of equilibrium between this force and magnetism when the 

 distance between the magnet ami the mass varies. 



4. To determine the general relation between force and distance, 

 tluit is to say, the law according to which the magnetic attraction 

 decreases v:hen the distance is increased. 



The first of these propositions has engaged the attention of 

 physicists from time to time during a considerable number of 

 years. Experiments have been made and facts multiplied, which, 

 however, are so disunited and contradictoiy as to render any 

 attempt to reduce them to a common law altogether hopeless. 



Tlie most important expei'iments which have been made in 

 connexion with this subject are those of Lenz and Jacobi. Bj-- 

 these exj)crimcnts it was established, that the attraction between 

 two electro-magnets, or an electro-magnet and a mass of soft 

 iron, is ])roportional to the square of the magnetizing streamf. 

 Here, howc\ er, an interval of about y'ljth of an inch separated 



* Coinmiiuicated by the Autlior. 

 + Poffgeiulorff's Aniwhn, vol. xlvii. p. -lO.'j. 

 Phil. Mag. S. \. \'ol. 1 . No. 1. April \m\. T 



