ELECTRO-MAGNETISM. 



length on a terminated current; that is, 

 on a portion of another current, situated 

 on one side only. The difficulty arises 

 from the impossibility of limiting the 

 actions to those parts of the currents 

 to which we wish to confine them, 

 while studying their effects, and of ex- 

 cluding the action of the remaining 

 portions of the currents necessary for 

 completing the circuit. The only mode 

 of preventing the interference of the 

 latter, is to neutralize them by opposing 

 one part to another ; this may in a great 

 measure be accomplished by providing 

 for the subdivision and branching off of 

 the currents in different directions, so 

 that their actions may destroy one an- 

 other. If, for instance, the ends of the 

 smaller wire that is to be rendered 

 moveable, be made to dip into a vessel 

 of mercury, of sufficient width to allow 

 of the electric currents to diverge and 

 spread over a considerable surface, they 

 will not materially interfere with the 

 actions we are examining. 



(207.) In the cases we have just 

 considered, ( 204,) the axis of ro- 

 tation in the shorter wire was sup- 

 posed to be at its extremity. If 

 its situation were different were it, 

 for example, in the middle of its length, 

 as at X, fig. 118, it is evident that when 

 the current is passing from' A to B, it 

 moves in the first portion towards the 



Fig. 118. 

 A 



XX 



B 



^D 



rotatory forces would oppose each other ; 

 and as the strongest would prevail, it 

 would bring the conductor into a position 

 where they are in equilibrium. Seey^. 

 119, where the portion /B of the limited 



centre, and in the second portion from 

 it; hence the rotatory forces, denoted 

 by the arrows a, b, counteract one an- 

 other, and the wire is urged to revolve 

 only by the difference, if there be any, 

 between them. 



(208.) If the current, which we have 

 hitherto supposed to be terminated, that 

 is, altogether on one side of the recti- 

 linear current, were prolonged so as to 

 cross the latter (without, however, join- 

 ing it), that portion which extended 

 beyond it would have a tendency to 

 move in an opposite direction to the 

 part on the other side; so that these 



Fig. 119. 



B 



rectilineal current A B, turning on the 

 axis A, and crossing the unlimited cur- 

 rent C D, tends to move in a contrary 

 direction to the portion A/, the position 

 of equilibrium being that of A E. 



(209.) If the current A B be traversed 

 by the current C D passing through the 

 axis itself, A, the position of equilibrium 

 is that of parallelism between the two 

 wires. In either case there can be no 

 revolution round the axis. 



$ 5. Action between Currents situated 

 in different Planes. 



(210.) Let us now consider what will 

 happen when the two currents A B and 

 C D, fig. 120, are in different planes, 

 both being of an indefinite length, and 

 extending on both sides of the perpen- 

 dicular line P Q, which is common to 

 the directions of both currents, and 

 divides each into two portions. At- 



120. 



traction will take place between those 

 portions in which the current is passing 

 towards P or Q, the points at which the 

 currents are nearest to each other ; that 

 is, in the present instance, between the 



