350 TENTH ANNUAL REPORT OF 



for comparison, are satisfactory. The want of accurate nuraerical de- 

 terminations occasions great uncertainty in regard to the advantages 

 and disadvantages of different galvanic combinations. If such uncer- 

 t-ainty exists in the accounts of men of science, it is not at all surpris- 

 ing to find communications in technical journals^ which betray entire 

 ignorance of the principles here discussed. 



Let us now examine the most important of the galvanic combina- 

 tions somewhat more closely. 



§ 15. The siniph zinc and copper hattery. — The Wollaston battery 

 is a convenient form of the simple zinc and copper combination, with 

 one liquid. 



The batteries of Young and Miinch may be considered as variations 

 of Wollaston's, and therefore a description of them is not necessary. 



The simple zinc and copper battery, it is well known, is not con- 

 stant, because the electro-motive force is considerably modified by the 

 polarization of the copper plate, which takes place in consequence of 

 the current. Poggendorff found, as we have seen, the electro-motive 

 force of the zinc and copper battery in dilute sulphuric acid, before 

 being modified by polarization, to be equal to 13.8, while the electro- 

 motive ibrce of Grove's battery is equal to 22.9. 



Assuming the electro-motive force of Grove's battery to be 830, re- 

 ferred to the chemical unit, (see table § 9,) the unmodified electro-motive 

 force of the zinc and copper battery would be 500 of the same unit. 

 But according to my experiments, when the current commences, the 

 electro-motive force of the zinc and copper combination is only 208 ; 

 thus, by polarization, the force is very soon reduced to f of its origi- 

 nal value, and this is also the reason that immediately after immer- 

 sion the current is exceedingly strong, but then very rapidly de- 

 creases. The polarization having once reached its maximum, the 

 current remains tolerably constant — at least, so much so as to admit of 

 accurate measurement. The numbers from which the values previ- 

 ously given (§ 9) of electro-motive force and of resistance to conduction 

 of WoHaston's battery were computed were not immediately observed, 

 but are the means of numerous readings. To form a correct idea of 

 the action of this battery, I will give here the corresponding series of 

 observations entire: 



Kind of wire inserted. Deflection. 



26° 



Copper 12 



,, : 11.5 



24 



Copper 11 



,, 11.25 



23.5 



Brass 5 



24 



Brass 5 



22 



.. 23 



Copper 11 



23 



