Barker.] OO^ l^jj^n 19^ 



falls to the bottom ; thus reversing the normal conditions in the battery. 

 In 1871 Sir Wm. Thomson attempted to reverse the position of the plates 

 in this gravity battery and place the zinc at the bottom in contact with 

 the heavier solution. But the collateral disadvantages arising from 

 the change more than balanced the advantages. He returned to the 

 old form, therefore, but arranged a siphon in such a way that the 

 zinc sulphate solution should be gradually withdrawn and too great 

 concentration avoided. In practice the zinc sulphate should never be 

 allowed to accumulate so as to increase the density of the solution above 

 1.17. This may be accomplished readily by pouring off the solution from 

 the top of the jar and replacing it by pure water. When freshly set up, 

 both of the torras of battery above described require to be kept on 

 closed circuit far a day or two. Their condition of equilibrium is then 

 reached and they may be used for the determination of electromotive 

 force. 



The difference of potential between the electrodes of a Daniell cell has 

 been determined by many experimenters; by Regnauld, by Poggendorft, by 

 Bufl[, by Beetz, by Petruschefsky, by Clark and Sabine, and by Ayrton 

 and Perry, among others. They find that while it varies somewhat under 

 variations of condition, yet tliat on the whole, it is remarkably constant, 

 the maximum being 1.031 and the minimum 0.901 volt. In all these ex- 

 periments the copper was immersed in a saturated solution of copper 

 sulphate. The zinc was placed in solution of sodium chloride, in dilute 

 sulphuric acid or in solution of zinc sulphate, all of varying strengths in 

 the different experiments. It is noticeaijle that in none of these measure- 

 ments made by indirect methods is the electromotive force as high as that 

 alreadj' mentioned as having been obtained by Sir William Thomson by 

 means of his absolute electrometer. Since the electromotive force of a 

 Daniell cell is the sum of the differences of contact-potential within it, it 

 would seem that any variation in the value of this electromotive force 

 must be due either to a change in the character or concentration of the so- 

 lutions, or to a difference of temperature. Moreover it has been observed 

 that the electromotive force of the gravity form of battery is always a lit- 

 tle higher than that of the cell in which a porous cup is used ; a result due, 

 probably, to the different conditions under which the diffusion of the two 

 liquids takes place, a fact pointed out by J. W. Draper in 1834. 



Using, therefore, the same form of battery, the solutions being always 

 the same in kind and in concentration, and the temperature being the 

 same, it is fair to infer that the Daniell cell may be made sufficiently con- 

 stant to serve as a reliable standard of electromotive force. Several 

 attempts have been made to do this. Raoult in 1864 (Ann. Chim. Phys., 

 IV, ii, 345), proposed a standard cell consisting of two covered jars of 

 glass, one containing a copper plate in a saturated solution of copper sul- 

 phate, the other a zinc plate in a solution of zinc sulphate in an equal 

 weight of water. The two were connected by an inverted U tube, whose 

 ends were closed by porous plates of earthenware cemented to them. By 



