Chemical Affinity in terms of Electromotive Force. 211 



wire fused into a piece of glass tubing, forming a ^ mercury- 

 cup; the soldering and the whole length of the platinum wire 

 being thickly covered with gutta percha, so that only zinc (or 

 copper) was exposed to the fluid. The plates were then con- 

 nected with a series of mercury-cups in such a way that by 

 simply moving a double switch connected at one end with two 

 mercury-cups in connexion with the electrometer-quadrants, 

 and dipping at the other end into two of the series of cups, 

 any required pair of zinc and copper plates could be brought 

 into connexion with the electrometer. Fig. 3 shows the 

 arrangement used for two pairs of plates, and fig. 4 that for 

 Fig. 3. Fig. 4. 



three pairs. In the former the two zinc plates, z l and z 2 , are 

 connected with mercury-cups Nos. 1 and 3 (No. 1 being also 

 connected with No. 5), whilst the two copper plates, c^and c 2 , 

 are connected with cups Nos. 2 and 4 respectively. Cups 

 Nos. 6 and 7 are connected with a standard cell (the error of 

 which, in reference to the average taken as T457 volt, is 

 known). By means of a double switch any pair of the series 

 of cups 1 to 7 can be connected with cups 8 and 9, which 

 are connected with the electrometer through the usual rever- 

 sing-gear; so that when Nos. 1 and 2, 2 and 3, 3 and 4, and 

 4 and 5 are thus connected, the electromotive forces due to 

 the combinations z 1 c 1 , c x z 2 , z 2 c 2 , c 2 z x are respectively read off; 

 whilst when 6 and 7 are connected (as represented in the 

 figure) the electrometer-scale is standardized. In actually 

 taking readings a double set was always employed, the switch 

 being successively used to connect the four combinations and 

 the standard with the electrometer, and then to connect them 

 Phil Mag. S. 5. Vol. 13. No. 81. April 1882. Z 



