ELECTRO-MAGNETISM. 



rent that acts upon it, and which it will 

 be convenient to designate a terminated 

 current. Thus, the current A B, fig. 

 Ill, lying* wholly on one side of the ex- 

 tended current C D, and which is there- 

 fore a terminated current, will be urged 



Fig. Ill 

 A 



JV1 



A. 



in the direction of the dotted line M R. 

 For since the current A B is moving 

 from the angular point B towards A, 

 while the current C D is, in the part 

 C B, moving towards B, the former will 

 be repelled by the latter during the 



whole of that part of its course. The 

 resultant of all the repulsive forces thus 

 operating on A may be represented by 

 the line M E. On the contrary, the 

 currents passing along the lines A B 

 and B D, are both moving in a direction 

 from the angular point B ; consequently 

 they attract one another; and the re- 

 sultant of these attracting forces may 

 be expressed by the line M F, which, 

 when combined with M E, gives the 

 total resultant M R, perpendicular to 

 AB. 



(200.) Applying this principle to 

 various positions of a terminated cur- 

 rent with relation to the extended one, 

 we obtain results which are expressed 

 in the annexed diagram, fig. 112; 

 where, as before, C D is the extended 

 current passing from C to D, and the 

 upper lines represent various positions 

 of the terminated currents, of which the 

 directions are marked by the terminal 

 arrows ; while the dotted arrows point 

 out the directions of the resulting mo- 



Fig. 112. 



tions. As a general rule, whenever the 

 direction of the terminated current is 

 from the line of the extended current, it 

 is urged to move in the same direction 

 as the extended current moves ; when 

 its current is towards the extended cur- 

 rent, it is urged to move in the contrary 

 direction. 



(201.) It is easy to perceive that, if, 

 instead of allowing the conducting wire 

 transmitting the terminated current to 

 obey its tendency to move parallel to 

 itself, its motion were restricted to ro- 

 tation round a fixed axis at one of its 

 extremities, as shown in fig. 113, the 

 force arising from the action of the in- 



Fig. 113. 



A 



O 



xr-.- 



X- >^.../ 



definite current C D, will carry the ter- 

 minated current round the whole cir- 

 cumference of the circle. This rotatory 

 force is independent of the angle of 

 inclination of the currents, and is, con- 

 sequently, uniform in every position of 

 A 13 ; and will accordingly act as an 

 uniformly accelerating force, causing 

 the wire to revolve with continually 

 increasing velocity, until checked by 

 friction and other mechanical obsta- 

 cles. 



(202.) The direction in which the wire 

 revolves depends, of course, upon the 

 relation between the directions of the 

 two currents concerned. "When the ter- 

 minated current is passing from the 

 centre to the circumference in the 

 shorter wire, its revolution will be as 

 represented in the figure ; that is, in a 

 direction contrary to that of the un- 

 limited current, in that part of the circle 

 which is nearest to it, but similar to it 

 in the more distant part of the circle. 

 The reverse takes place when the cur- 

 rent in A B is passing from the circum- 

 ference to the centre. 



(203.) While such is the action of the 



