24 



ELECTRO-MAGNETISM. 



pleted by means of a thick copper wire 

 proceeding from the bottom of the vessel, 

 coming out through the side, and termi- 

 nating in another cup holding a small 

 quantity of mercury, by which a com- 

 munication may be established with the 

 other pole of the battery. As soon as 

 this connexion is effected, the magnet 

 begins to revolve round its axis with 

 great rapidity, the rotation continuing 

 as long as the connexions with the bat- 

 tery are preserved and the battery retains 

 its power. 



(69.) In the original experiment of 

 Ampere, the magnet was allowed to 

 float without support in the mercury, 

 being kept in a vertical position by a 

 weight of platina attached to its lower 

 end. But this addition to the whole 

 mass to be moved occasions a great 

 diminution of effect, so that the appa- 

 ratus above described gives a much 

 greater velocity of motion with the 

 same galvanic power. 



(70.) The same phenomenon has been 

 exhibited in various ways; the principle 

 on which it depends is that the electric 

 current should descend through the 

 upper half of the magnet only, so as to 

 act exclusively on the pole which is 

 situated in that half, and afterwards be 

 diverted from the magnet, and made to 

 pass away in such a direction as that it 

 shall not affect the lower pole of the 

 magnet. In the experiment above re- 

 lated, the electric current, after travers- 



ing the upper half of the magnet, passes 

 into the mercury, and being diffused 

 through it, acts in no sensible degree 

 on the lower pole of the magnet, and 

 does not interfere with the rotation pro- 

 duced by its influence on the upper pole. 

 There are several circumstances, how- 

 ever, to be taken into account, in ex- 

 plaining this experiment, which cannot 

 now be easily rendered intelligible, and 

 the notice of which must be reserved 

 for a future part of the Treatise. 



(71.) The same object is attained in 

 the following manner, by an apparatus 

 represented in Jig. 43, and in section 

 in Jig. 44. A magnet, pointed at both 

 ends, is supported below by an agate 

 cup fixed on a stem rising -.from the 

 bottom of the stand ; while its upper 

 point is lightly pressed upon by a screw, 

 with a milled head, passing through a 

 screwed hole at the top of an arched 

 beam, which forms part of the sustain- 

 ing frame- work of the apparatus. Near 

 the middle of the magnet, this frame 

 supports a stage in the form of a ring, 

 through the centre of which the magnet 

 passes freely, and carrying a circular 

 cistern of mercury, which also sur- 

 rounds the magnet, without touching 

 it. A similar cistern of mercury sur- 

 rounds the lower stem, which supports 

 the agate cup. A copper wire, project- 

 ing into the interior of each of these 

 cisterns, passes out through its sides, 

 and, being bent upwards, terminates in 

 a small cup, holding a little mercury, for 



Fig. 44. 



