[ 67 j 

 XI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from vol. ii. p. 568.] 

 Dec. 11, HPHE reading of Dr. Faraday's paper, entitled " Experi- 

 1851. -■- mental Researches in Electricity. Twenty-eighth 

 Series. On Lines of Magnetic Force ; their definite character ; and 

 their distribution within a Magnet and through Space," which was 

 commenced on the 27th November, was resumed and concluded. 



The author defines a line of magnetic force to be that described 

 by a very small magnetic needle, when it is so moved, in either 

 direction correspondent to its length, as to remain constantly a 

 tangent to the line of motion ; or as that along which if a transverse 

 wire be moved in either direction, there is no tendency to the forma- 

 tion of an electric current in the wire, whilst if moved in any other 

 direction there is such a tendency. Such lines are indicated by 

 iron filings sprinkled about a magnet. These lines have a deter- 

 minate direction ; they have opposite qualities in and about this di- 

 rection, and the forces in any part of them are determinate for a given 

 magnet. They may, as the author thinks, be employed with great 

 advantage to represent the magnetic force as to its nature, condi- 

 tion, direction, and comparative amount; and that in many cases 

 when other representations of the force, as centres of action, will 

 not apply. 



The term line of force, as defined above, is restricted to mean no 

 more than the condition of the force in a given place, as to strength 

 and direction ; and not to include any idea of the nature of the 

 physical cause of the phenomena : at the same time if reason should 

 arise to think that the physical condition of the force partakes ge- 

 nerally of the nature of a current or of a ray, a view which the 

 author inclines to, he sees no objection in the term, any more than 

 to the terms current and ray, as they are used in considerations 

 regarding electricity and light, because it may accord with such a 

 view. 



The lines of magnetic force, as defined above, may be recognized 

 either by a magnetic needle or by a moving wire ; but the two 

 methods are founded on very different conditions and actions of the 

 magnetic force, and the moving wire appears to have the largest 

 application. Its principle can be applied in places which are inac- 

 cessible to the needle, and it can sum up the forces in a given plane 

 or surface at any distance from the central magnet. It has no re- 

 ference to results of attraction or repulsion, and in some cases is 

 opposed to them ; but the author thinks it gives a true view of the 

 disposition of the magnetic powers, and leads, and will lead to a 

 more correct understanding of the nature of the force. For these 

 reasons he advocates its adoption, not to the exclusion of the needle, 

 but in conjunction with it ; and proceeds to devclope the experi- 

 mental methods and their results, and first in the case of a bar 

 magnet. 



Two bar magnets, each 12 inches long, 1 inch in width, and 0'4 

 F2 



