182 APPENDIX TO MEMOIR OF PELTIER. 



ported by the point of the pivot resting in the steel cup, is suspended by the 

 silver wire, without being, however, in njetallic contact with it, since it is sepa- 

 rated from it by the small cylinder of gum-lac. 



When these changes have been made, the electrometer has become a torsion 

 balance ; only, if we wish to use it as such, it is necessary to withdraw from 

 the indicating needle the small wire of magnetized steel, or still better, to have 

 a spare needle for exchange. There remains but one other condition to fulfil in 

 order that the torsion balance should be completely prepared ; that is, to estab- 

 lish the communication between the needle and the capsule, though with the 

 exclusion of all friction. 



In the torsion balances, Peltier employs acidulated water, because the> point 

 which descends into the liquid is of platina; but here, as the point is a pivot of 

 steel, even pure Avater cannot be used, much less acidulated water, for the pivot 

 would be soon oxidized. Doubtless this communication might be established 

 by means of mercury poured into the little cup ; but this metal is too resistant, 

 and detracts much from the sensibility of the instrument. There is, besides, an 

 inconvenience in using it; its resistance prevents the needle from placing itself 

 perfectly at its centre of gravity ; wl.ence it results that the suspending wire, 

 instead of being vertical, has a slight inclination, and consequently the needle 

 has a tendency to fall to one side. The liquid which suits best is a solution of 

 potash, for this preserves unimpaired the polish of iron and steel, and suffices as 

 a conductor for the electricity of tension between two bodies in such close prox- 

 imity as the steel cup and its pivot. 



At first Peltier had given to his electrometer dimensions somewhat large. It 

 was then, in effect, a cabinet instrument ; but afterwards, when he occupied him- 

 self with meteorology, he perceived the necessity of reducing these dimensions, 

 in order to render it more manageable and portable ; he therefore constructed 

 an electrometer of small size and very nearly conformed to the proportions of 

 an ordinary electroscope. This instrument has been also adjusted to the use for 

 which it was to serve. The fixed rod no longer communicates outside laterally 

 and by the foot-stand ; its interior extremity, that which is above the centre of 

 the dial, is cun^ed from below upwards, and issues from the casing by its upper 

 wall; it is then prolonged vertically for two decimetres, and is surmounted by 

 a hollow metallic ball, eight centimetres in diameter. This is the atmospheric 

 electrometer of Peltier. 



We must not quit this subject without mentioning that these electrometers all 

 require that a table giving the ratio of the forces to the arc of deviation should 

 be constnicted for each of them. It is the same, in effect, with electrometers 

 as with galvanometers : their angular deviation is not proportional to the forces. 



III. — Dynamic electeicitt. — voltaic pile. 



Of the pile of Volta and the theory of contact. — The most usual source of 

 dynamic electricity is the pile of Volta. This is one of the most admirable 

 instruments w'ith Avhich the genius of man lias enriched science, and numerous 

 physicists have occupied themselves with its theory. 



Volta supposed that at the contact of two heterogeneous metals, there is a 

 force which constantly decomposes their natural electricity ; that this force pro- 

 jects on the one side positive and on the other negative clectricit}' ; that the 

 interposed liquid serves only as a conductor to allow" the recombination in the 

 neutral fiuid of the two opposite cuiTcnts. It was this decomposing power 

 placed at the contact of the metals that he called the electro-motive force. This 

 theory has received the name of the theory of contact. 



According to this theory, the liquid acts but as a conductor ; an experiment 

 of Davy's, however, soon evinced the inexactness of this assertion. After hav- 

 ing constructed a battery of cups, of copper and iron, Davy fii'st poiu'ed pure 



