182 APPENDIX TO MEMOIR OF PELTIER. 



ported by tlie point of the pivot resting in tlie steel cnp, is suspended Ly tlie 

 bilver wire, without heing', however, in metallic c<;ntact v;ith it, since it is sepa- 

 ]'ated from it by the small cylinder of gum-hic. 



When these chang'es 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 Avire 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 completel}^ 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 water cannot be used, much less acidulatect water, for the pivot 

 would be soon oxidized. Doubtless this communication might be estaldished 

 by means of mercury poured into the little cuj) ; 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 ])lacing itself 

 perfectly at its centre of gravity; M-J.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 sutHces as 

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

 imity as the steel cup and its i)ivot. 



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 lixed rod no longer communicates outside laterally 

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

 the dial, is curved from below upwards, and issues from the casing l>y its upper 

 wall; it is then prolonged verticall}- for two decimetres, and is surmounted by 

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

 electrometer of Peltier. 



We nuist 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 constructed 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. — DYiVAillC ELECTRICITY. — VOLTAIC PILE. 



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

 dynamic electricitj'- is the pile of Volta. This is one of the most admu'able 

 instruments Avith which the genius of man has enriched science, and numerous 

 physicists have occupied themselves with its theory. 



Volra sup})osed 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 electricity; that the 

 interposetl liquid serves only as a conductor to allow the recombination in the 

 neutral fluid 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 theor^^ of contact. 



According to this theor}", 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 first poured pure 



