THE SMITHSONIAN INSTITUTION. 341 



lias multiplied tlie roots of the torsions by 1.0230, the values deter- 

 mined hy the angle of deflection by 1.813G, by which means the re- 

 sults b)' the first experiment arc rendered periectly accordant. But 

 siucc the rest of the corresponding numbers accord very closely, these 

 experimental series prove that the dectro-moiiv'e force is jproportional 

 to the clecfroscopic tension at the poles of the open battery. 



This principle might be proved with less sensitive electrometers, by 

 determining the tension at the poles of a battery of 30, 40, or more, 

 elements. 



Kohlrauscli's instrument is also very well adapted to solve a dis- 

 puted theoretical question, to which allusioji has been made above. 

 If a strip of zinc and one of platinum be immersed in a vessel of water 

 without touching each other^ according to Schonbein's view, the upper 

 end of the zinc must indicate free negative electricity — the upper end 

 of the platinum, free positive ; while according to the contact theory the 

 reverse should be the case. It is very desirable that Kohlrausch himself 

 should investigate this, because he not only possesses an excellent 

 instrument of the kind, but has attained great skill in manipulating 

 with the apparatus, 



§ 12. Indirect methods for determining the constants of the lattery. — 

 The process given above, derived from formulas (1) and (2), for de- 

 termining the resistance and electro-motive force of a galvanic battery, 

 and that for determining the constants, which we will call Ohm's 

 method, is as simple as it is accurate, if a suitable measuring appa- 

 ratus is furnished, and a battery sufficiently constant be used. Both, 

 however, were wanting at the time of the publication of Ohm's law, 

 and it thus happened that complicated methods had to be used to ob- 

 tain only tolerably accordant results. By degrees only, simplicity was 

 attained in this instance, as is often the case in the history of physics. 



First, there was wanting an instrument adapted to measuring the 

 force of current ; then the multipliers used were objectionable in two 

 particulars : they were suited for weak currents only, and there was 

 no simple law, showing the relation of the angle of deflection and the 

 force of the current. 



Several physicists have proposed very ingenious methods for gradu- 

 ating a galvanometer ; that is, to determine empirically what rela- 

 tion the different degrees of deflection have to the force of current; yet 

 since they do not appear to be very well adapted for general use, and 

 only yield useful results in the hands of skilful experimenters, I may 

 be pardoned for not going into the details of these methods of gradu- 

 ating. The method v/hich Poggendorff has given for converting the 

 galvanometer into a measuring instrument, is found in volume LVI 

 of his Annalen, page 324. There is also in this paper a short col- 

 lection of the methods recommended by other physicists for the 

 same purpose, with indications of the sources, to which I must refer 

 those who v/ish to enter into the details of tliis subject. 



Fechner did not use the deflection of the needle for determining 

 the force of current, but the period of oscillation of the needle about 

 its position of equilibrium, for the case in which the coils of the mul- 

 tiplier are parallel to this position. 



