524 JAMES CLERK MAXWELL. 



and the other a south pole. Such a disc will act upon 

 external magnets in the same manner as the current if it be 

 magnetised, so that a right-handed screw rotating with the 

 current would enter at the south face and emerge at the 

 north face. Such a magnetised disc is called a magnetic 

 shell, and it will of course be acted upon by a magnet with 

 forces exactly equal and opposite to those with which the 

 magnet is acted upon by it. The magnetic lines of force 

 proceeding from a circuit conveying an electric current are 

 therefore the same as would proceed from the magnetic 

 shell above described, the strength of the magnetisation 

 being properly adjusted ; in other words, the magnetic field 

 around such a circuit is the same as that surrounding the 

 magnetic shell, and hence it follows that two circuits, each 

 conveying electric currents, will act upon one another in the 

 same way as two magnetic shells whose circumferences 

 coincide with the wires, and which are magnetised as 

 above described. 



Now if the shells be parallel and magnetised in the 

 same direction, they will have their opposite faces presented 

 towards each other, and will attract one another. If they 

 are magnetised in the opposite directions they will repel 

 one another. Similarly, two parallel circuits will attract 

 one another if the currents be passing in the same direction 

 in both, and will repel one another if they be going in 

 opposite directions. Also two parallel wires, which may be 

 considered as parts of such circuits, will attract one another 

 when the currents in them are going in the same direction, 

 and repel one another if they are going in the opposite 

 directions. Maxwell's rule for determining the manner in 

 which a circuit conveying a current will behave in the 

 presence of other currents or of magnets is a very simple 

 expression of Faraday's results. Defining the positive direc- 

 tion through a circuit as that in which a right-handed screw 

 would advance if rotating with the current, he enunciated 

 the rule thus : 



If a wire conveying a current be free to move in a mag- 

 netic field it will tend to set itself so that the greatest possible 



