CONTRIBUTIONS TO SCIENCE. 521 



which the magnets are composed is sufficiently hard to pre- 

 vent the actions of the magnets on each other altering the 

 strengths of their poles. 



If a sheet of paper be supported horizontally above the 

 poles of a magnet, and iron filings be sprinkled over the 

 paper, each filing becomes magnetised by induction in the 

 direction of the resultant magnetic force at the point where 

 it is situated, and if the paper be gently tapped so as to 

 overcome friction, the mutual attraction of the unlike poles 

 in the filings causes them to adhere together in threads 

 or filaments, the North pole of one filing attaching itself to 

 the South pole of a neighbouring filing, and so on, the points 

 of attachment all lying along a line of force. In this way 

 the filings form a graphic representation of the lines of mag- 

 netic force, and it was this experiment which first suggested 

 to Faraday the idea of the physical existence of such lines ; 

 and as he found it difficult to conceive of curved lines of 

 force being due to " direct action at a distance " (Exp. Kes. 

 1166), he considered that there must be some medium which 

 is the vehicle both of magnetic and electric forces, and that 

 such forces are propagated from particle to particle of the 

 medium. Faraday also supposed that the same medium 

 might serve as the vehicle for the transmission of light. 

 The investigation of the properties of the medium necessary 

 to account for observed electric and magnetic actions, the 

 explanation of these actions, and the determination of the 

 velocity of light from purely electro-magnetic considerations 

 on the hypothesis of the existence of a such a medium con- 

 stitute Maxwell's greatest contribution to electrical science. 



The action of an electric current upon a magnet was first 

 observed by (Ersted. It is said that he made many attempts 

 in his laboratory to discover an action between a magnet and 

 a wire conveying a current, but in all his attempts he care- 

 fully placed the wire at right angles to the magnetic needle, 

 and could detect no effect whatever. On attempting to 

 repeat the experiment in the presence of his class he placed 

 the wire parallel to the needle, and the latter immediately 

 swung round and ultimately came to rest nearly at right 



