

TRANSACTIONS OP THE SECTIONS. 29 



Let us then take a piece of iron, and after having magnetized it, by causing trans- 

 itory electric currents to circulate around it, let us treat it with contrary and weaker 

 currents than the preceding, until it no longer presents any polarity. If with the 

 said contrary currents I produced in the iron polarities which did not mutually de- 

 stroy, but only neutralized each other, what should happen if the iron were subjected 

 to some shock ? This act destroys a greater portion of the weak polarities than of 

 the strong, and therefore the iron after the shock will appear possessed of some de- 

 gree of magnetism, in the direction in which it had been more strongly magnetized. 

 And this is conformable to what the experiment shows. 



An iron cylinder, nine centimetres in length and weighing fifteen grammes, after 

 being magnetized so that the needle of the magnetometer deviated 60°, was deprived 

 of all polarity by means of several electric currents, so that the needle pointed to 

 zero. After this, having let the iron fall from the height of two metres upon the 

 pavement, it recovered such magnetic force that the needle deviated 16°. 



If, instead of acting with moderate or weak currents to neutralize entirely the po- 

 larity, some little is still permitted to remain, an iron is obtained, which has only 

 force to cause the needle to deviate 6 or 7 degrees ; but after the shock the needle de- 

 viated 16 or 17. 



And if the weak and contrary magnetizations are made use of until the iron causes 

 the needle to deviate 4 or 5 degrees in a contrary direction (that is, towards the east, 

 if it first deviated towards the west), then is the iron in such a state, that, by means 

 of the shock, it changes polarity, that is, presents the south pole at the extremity 

 which had the north, and vice versa. 



These facts, which may be varied in many ways, prove, if I err not, that in a 

 piece of iron treated as I have mentioned, there exist systems of magnetic force, 

 which altogether or in part neutralize each other. 



Let us proceed now to describe some experiments which show the truth of the other 

 two propositions. 



An iron wire eight centimetres long, and weighing 118 decigrammes, was mag- 

 netized until the needle of the magnetometer deviated 12°. A small Leyden jar 

 (one square decimetre of coated surface), charged to the tension of 10° of the elec- 

 trometer on the double quadrant of Volta, was discharged upon the coil in which 

 was the iron, and so as to magnetize it in the same direction in which it was already, 

 and the effect was that the needle of the instrument went from 12° to 25°. 



Having destroyed the magnetism of the said iron by means of shocks, and then 

 magnetized until the needle of the magnetometer remained at 59°, and having dis- 

 charged as above upon the coil the small jar charged to 10 degrees of tension, the 

 needle moved from 59° to 63°. 



Whence it is seen, that the more the iron is magnetized the less a given current 

 can strengthen its magnetism. 



I magnetized another iron equal to the preceding, so that it made the needle 

 deviate 9°, and after having discharged upon the coil which contained the iron the 

 jar charged to the usual tension, but so that it might tend to magnetize it oppo- 

 sitely, the needle passed from the degree 9°, which it marked on one part to the 

 1° on the opposite part. 



Having magnetized the iron again so that the needle deviated to 20°, and having 

 afterwards discharged upon the coil the jar with its usual tension, and in a con- 

 trary direction, the needle moved from the 20° to the 10°. 



Magnetism being also taken from the iron which then remained, then magnetized 

 so that the needle remained at 60°, and then having caused to circulate round it the 

 usual current excited by the small jar charged to 10° of tension, and so as to pro- 

 duce a contrary magnetism, the needle moved from 60° and stopped at 44°. 



Thus, the more a piece of iron is magnetized, the less is its magnetism neutralized 

 by a contrary current. 



Another piece of iron similar to the preceding being magnetized so that the 

 needle was fixed at 29°, and the jar with the usual tension being then discharged, 

 and so as to strengthen the magnetism already possessed by the iron, the needle 

 moved from 29° to 41°. 



Having, by the usual mechanical operations, destroyed the magnetism of this iron, 

 then magnetized so that the needle was at 27°, upon the discharge of the jar with 



