32 



ELECTRO-MAGNETISM. 



any moment, by removing one end of 

 either connecting wire from its cup, or 

 by replacing it. The direction of the 

 current may also be readily changed, by 

 merely exchanging the situation of the 

 wires in two of the cups. The extre- 

 mities of the connecting wires should 

 be made perfectly bright, and the ends 

 of the wire arms which support the 

 cups and enter the mercury in them, 

 ought also to be in a similar state, so 

 that a perfect metallic contact may be 

 preserved. 



(87.) In making electro-magnetic ex- 

 periments, where numerous repetitions 

 of contacts between wires are often re- 

 quired, it is extremely useful, if these 

 wires are of copper, to rub the ends 

 over with a little nitrate of mercury ; an 

 amalgam is thus formed on the surface 

 of the copper, which does not oxidate or 

 become dirty, as copper itself does, but 

 remains bright, and fit for voltaic con- 

 tact for a considerable length of time. 

 For this useful manipulation we are in- 

 debted to Mr. Faraday. 



(88.) The movement of currents by 

 the influence of the pole of a magnet 

 may be exemplified in fluid as well as 

 in solid conductors. Thus mercury, 

 while conducting a current of electricity, 

 is made to exhibit these motions with 

 the greatest facility. By immersing the 

 points of the positive and negative wires 

 into a shallow basin containing mercury, 

 a magnet held either above or below 

 the line of communication will cause 

 the mercury to revolve round the points 

 from which the currents diverge. This 

 motion may be rendered more evident 

 by covering the mercury with a very di- 

 lute acid solution, which occasions the 

 disengagement of bubbles of air which 

 are moved along with the mercury. The 

 same phenomenon may also be exhibited 

 in the following manner. If the positive 

 wire terminate in a steel point which is 

 dipped into mercury contained in a shal- 

 low basin, so as to convey into it an 

 electric current, which, passing in radi- 

 ating lines through the mercury, is re- 

 ceived by a copper ring surrounding the 

 steel point, and so tranferred to the ne- 

 gative pole, by placing the pole of a 

 strong magnet underneath the basin 

 immediately below the steel-pointed 

 wire, the mercury will be seen to revolve 

 rapidly in a vortex round the point from 

 which the currents diverge. The revo- 

 lution is in the contrary direction, if 

 either the direction of the current be 

 reversed, or the opposite pole of the 

 magnet be applied. 



(89.) Sir Humphry Davy found that 

 the arched stream of electrical light 

 which extends between two points of 

 charcoal that are placed in the voltaic 

 circuit, as described in the Treatise on 

 GALVANISM, 27, is thrown into a rapid 

 rotatory motion by the action of the pole 

 of a magnet placed near it *. 



CHAPTER VII. 

 Concentration of Effects. 



(90.) We have already seen, 82, that 

 when a conducting wire is placed be- 

 tween the contrary poles of a magnet, it 

 receives a similar influence from these 

 poles, and is urged to move in one particu- 

 lar direction by the united force of both. 

 A similar combination of powers will 

 occur when the pole of a magnet is 

 placed between two parallel conducting 

 wires, in which the electric currents are 

 moving in opposite directions. Thus, if 

 the needle N S, Jig. 58, balanced as a 



Fig. 58. 



dipping needle, be placed between the 

 two wires W, w, in the former of which 

 the current is ascending, and in the lat- 

 ter descending, the north pole of the 

 needle will be urged in the same direc- 

 tion, denoted by the arrow a, by both 

 the wires, in a plane parallel to the 

 wires, and at right angles to the plane 

 in which they are both situated. The 

 south pole will also be urged in the con- 

 trary direction by both wires ; and the 

 needle will, by the combination of these 

 forces, have a strong tendency to turn 

 upon its centre. 



(91.) If the wires be joined together 

 at either end, or, what comes to the 

 same thing, if a continuous wire be bent 

 back upon itself, an electric current 



* Philosophical Transactions for 1821, p. 427. 



