ON PHYSICAL LINES OF FORCE. 463 



will be circles drawn in the opposite direction from that of the hands of a 

 watch ; that is, in the direction mvse. At e the lines of force Fig. 6. 



will be the sum of those of the field and of the current, and 

 at w they will be the difference of the two sets of lines ; so 



that the vortices on the east side of the current will be more 

 powerful than those on the west side. Both sets of vortices have 



5> J 



their equatorial parts turned towards C, so that they tend to 

 expand towards (7, but those on the east side have the greatest 

 effect, so that the resultant effect on the current is to urge it towards the west. 



The fourth term, 



may be interpreted in the same way, and indicates that a current q in the 

 direction of y, that is, to the north, placed in a magnetic field in which the 

 lines are vertically upwards in the direction of z, will be urged towards the east. 



The fifth term, 



- 

 dx" 



merely implies that the element will be urged in the direction in which the 

 hydrostatic pressure p l diminishes. 



We may now write down the expressions for the components of the resultant 

 force on an element of the medium per unit of volume, thus : 



< 13 >' 



The first term of each expression refers to the force acting on magnetic 

 poles. 



The second term to the action on bodies capable of magnetism by induction. 

 The third and fourth terms to the force acting on electric currents. 

 And the fifth to the effect of simple pressure. 



