522 JAMES CLERK MAXWELL. 



angles to the wire. Whenever the North pole (i.e. the North 

 seeking pole) of a magnet is brought near to a wire conveying 

 a current, the pole tends to go round the wire in a certain 

 direction, while the South (or South seeking) pole of the 

 magnet tends to go round the wire in the opposite direction, 

 and hence if the magnet be free to turn about its centre, the 

 magnet will come to rest at right angles to the wire. Many 

 memoriae technicce have been given for determining the manner 

 in which a magnet will behave in the neighbourhood of a 

 current. Maxwell's rule was as follows : Suppose a right- 

 handed screw to be advancing in the direction of the current, 

 and of necessity rotating as it advances, as if it were piercing 

 a solid. The North pole of a magnet will always tend to 

 move round the wire conveying the current in the direction 

 in which such a screw rotates, while the South pole will 

 tend to move in the opposite direction. 



We may thus suppose every wire conveying a current 

 to be surrounded by lines of magnetic force which form 

 closed curves around the wire, and the direction of the 

 force is that in which a right-handed screw would rotate if 

 advancing with the current. In the case of a straight wire 

 of infinite length, these curves are of course circles. Since 

 action and reaction are equal and opposite, it follows that 

 whatever be the mechanical force exerted by a current upon 

 a pole of a magnet, the latter will always exert an equal and 

 opposite force upon the wire or other conductor conveying 

 the current. Many experiments have been devised to show 

 this. Maxwell used to illustrate it in a very simple way. 

 Having attached a piece of insulated copper wire to a small 

 round plate of copper, he placed the plate at the bottom of a 

 small beaker. A disc of sheet zinc was then cut of such 

 size as to fit loosely in the beaker, a small " tail " of zinc 

 being left attached to it ; this was bent up and united to the 

 copper wire above the top of the beaker, while the plate of 

 zinc was suspended in a horizontal position an inch or two 

 above the copper plate. The beaker was filled up with dilute 

 sulphuric acid and placed on one pole of an electromagnet, 

 some sawdust or powdered resin being placed in the liquid 



