ELECTRO-MAGNETISM. 



time, by an apparatus contrived by Mr. 

 Faraday, in which the cups employed 

 in the two first experiments are both 

 acted upon by the same voltaic battery. 

 This compound apparatus is shown in 

 Jig. 40. The cups in which the lower 

 wires proceeding from the bottom of 

 each cup respectively terminate, are 

 made to communicate by means of 

 wires with the opposite poles of a bat- 

 tery. The upper wires communicate 

 by a cross wire, supported by an upright 

 pillar fixed in the middle of the stand. 

 The current of electricity, therefore, will 

 ascend through the mere iry and wire in 

 one of the cups, and descend in the other, 

 and produce at the same moment a re- 

 volving motion of the magnet in the one 

 case, and in the other case, a revolution 

 in the moveable wire. A cup is also 

 placed over the middle of the cross wire 

 lor the convenience of sending the elec- 

 tric current in the same direction along 

 both the wires, by making it communi- 

 cate with one of the poles of the voltaic 

 battery, while the lower cups both com- 

 municate with the other pole. The adop- 

 tion of this arrangement will produce a 

 corresponding change in the direction 

 of one of the rotations. 

 Fig, 41. 



(67.) The two phenomena may even 

 be shown in the same vessel, if, in that 

 containing the moveable magnet, y?g\ 33, 

 the wire which dips into the mercury be 

 rendered moveable, as in fig. 38, by a 

 mode of suspension adapted to that 

 purpose. The wire and the magnet will 

 then both revolve in the same direction 

 round a common centre of motion, each 



appearing to pursue and be pursued by 

 the other round the circumference of the 

 circle described by their revolution. 

 (See/o-,41.) 



(68.) After the discovery of the revo- 

 lution of a magnet round a conducting 

 wire, and of the wire round a magnet, 

 many attempts were made to obtain the 

 rotation of a magnet, or of a conductor, 

 round their own axes. Ampere was the 

 first who accomplished the former of 

 these objects, which may be effected by 

 the following method : The cylindrical 

 magnet seen in the section, /#. 4 2, termi- 

 nates at its lower extremity in a sharp 



steel point, which rests in the centre of a 

 conical cavity of agate, in the bottom of 

 the vessel, which may be either of glass 

 or wood. The upper end of the magnet 

 is supported in a perpendicular position, 

 by a thin slip of wood, passing across 

 the upper part of the vessel and resting 

 against its sides, having a hole through 

 which the magnet passes freely. A 

 piece of quill is fitted on the upper ex- 

 tremity of the magnet, so as to form a 

 cup or reservoir above it for receiving a 

 small quantity of mercury. Into this 

 mercury is inserted the lower end of 

 a wire which is amalgamated, in order 

 to obtain a perfect metallic contact, 

 while its upper end terminates in a cup 

 holding a globule of mercury, for the 

 purpose of forming a communication 

 with one of the poles of the voltaic bat- 

 tery. The vessel being filled with mer- 

 cury, so as to cover the lower half of 

 the magnet, the galvanic circuit is com- 



