ELECTRO-MAGNETISM. 



71 



(215.) Whatever be the action which 

 the circular current receives from the 

 rectilineal one, a similar and opposite 

 action is exerted by it on the latter, which 

 is urged to assume a position in the'plane 

 of the circle, and such that the adjacent 

 currents in each may be moving in 

 similar directions. 



(216.) The action of a circular cur- 

 rent upon a rectilinear, but terminated 

 current, situated wholly on one side of 

 the plane of the circle, and inclined to it 

 at a given angle, requires especial notice. 

 If the direction of the straight Current, 

 when prolonged, pass near ihe centre of 

 the circle, the forces that act upon it 

 are nearly balanced, and neither action 

 nor reaction is perceptible. If it be 

 near the circumference, the action of the 

 adjacent portion of that circumference 

 will predominate, and effects, similar to 

 those taking place between a terminated 

 and an unlimited current, will be pro- 

 duced, with this modification, however, 

 that the unlimited current being circular, 

 the motion of translation in a straight 

 conductor at right angles to the plane of 

 the circle, following the course of the 

 circumference, becomes itself circular ; 

 and if the conductor be attached to an 

 axis perpendicular to the plane of the 

 circle, and passing through its centre, 

 that conductor will be made to revolve 

 continually around that axis, as shown in 

 fig. 1 2 6 , where C D is the circular current, 

 and A B the straight moveable, but ter- 

 minated current. This rotatory force 



Fig. 126. 



extends its influence beyond the interior 

 of the circle to any distance, provided 

 the straight current do not pass beyond 

 the plane of the circle. The reaction of 

 the straight current on the circular con- 

 ductor impels the latter to revolve in thu 



contrary direction, as marked by the 

 dotted arrows parallel to it. 



(217.) All these effects will be con- 

 siderably increased if a great number of 

 similar currents be moving in the same 

 direction in the different parts of the 

 circuit, described by the straight current 

 in the last paragraph. This may be 

 obtained by making currents traverse 

 a number of wires placed in the surface 

 of a cylinder, and parallel to its axis, 

 which is also that of the rotation ; or, 

 what will be equivalent to this arrange- 

 ment, by making a current pass along 

 the surface of a hollow conducting cylin- 

 der, in the direction of its length ; for in 

 that case, the current may be considered 

 as dividing itself into an infinite number 

 of parallel filaments. 



(218.) A similar augmentation of 

 power may be obtained by multiplying 

 the circular currents, either by employ- 

 ing a wire coiled into the form of a ring, 

 or into that of a flat spiral. When these 

 rings or spirals are combined with the 

 cylinders above mentioned, the effects 

 are again proportionably increased. 



(219.) The modes of exemplifying 

 these conclusions experimentally are 

 various. Thus, a wire, as shown in fig. 

 127, consisting of two vertical branches, 

 united above by a transverse arch, to 



Fig. 127. 



the centre of which is affixed a steel 

 point turning downwards, for the pur- 

 pose of suspension, may be united below 

 to a circular rim, which dips into a 

 shallow trough of mercury, so as to en- 

 able us to transmit currents through the 

 wire, that will move in both the vertical 

 wires in the same direction; that is, 

 either upwards or downwards in both. 

 If, while so suspended and connected, a 

 circular current be made to act upon 

 it from below, whether by means of a 

 single circle, as shown in trie same 

 figure, or by a spiral coil, as that of 

 fig. 128, the wire will revolve round its 

 axis of suspension, in a direction deter- 



