Chemical Affinity in terms of Electromotive Force. 291 







Two opposed 



Cells. 







Total resist- 

 ance, in ohms. 



Scale-readings. 



Current, in 

 microamperes. 



E.M.F. 



in 

 milli- 

 volts. 



Fall in 

 E.M.F. 



First 

 series. 



Reversed 

 series. 



Mean. 



200,820 

 150,820 

 L00,820 



69-3 



92-1 



137-0 



70-1 



93-25 



138-0 



69-7 -060897 



92-67 1 -080965 



137-5 -120134 



12-23 

 12-21 

 12-11 



02 

 •11 



Hence with an increase of current from *06 to '08 micro- 

 ampere, representing an increase in density from '02 to '027, 

 the fall in E.M.F. was barely measurable, being only '02 

 millivolt ; but when the current rose to '12 microampere, 

 giving a density of *04, the fall in E.M.F. was distinct, being 

 upwards of '1 millivolt. 



151. In the same kind of way various other observations 

 were made, and indicated on the whole that the effect of 

 polarization in cells set up w T ith amalgamated-zinc and electro- 

 copper plates is not greater than about % 2 millivolt, as long 

 as the current-density does not exceed about '05 microampere 

 per square centimetre of plate-surface. 



Limiting values not far from this were similarly obtained with 

 other kinds of zinc- and copper-plate surfaces. Accordingly 

 in the observations made with these kinds of cells for the pur- 

 pose of estimating the variations in E.M.F. due to difference 

 in solution-strength, the current-densities were never allowed 

 to exceed this limit, and were usually a long way below it. 



In precisely the same kind of way the limiting values for 

 various kinds of diffusion-cells were arrived at. The following 

 table represents some of the average results obtained, the 

 E.M.F. 's being expressed in millivolts: — 



1 



Two opposed 



Diffusion-cells. 









Current- density, 





Zinc sulphate. 



Copper 





microamperes per 

 square centimetre. 



Amalgamated 



zinc and 

 electro-copper 



sulphate. 



Electro- 



Amalgamated- 



Electro- 





plates. 



zinc plates. 



zinc plates. 



copper plates. 



Not exceeding *02 



Below 0-02 



Below -03 



"1 Inappreciable 



Inappreciable 



•02 to -035 

 •035 „ -06 



„ o-i 



•1 to -3 



„ 1 



•1 to -2 



I Below -1 



j Below -1 



•06 „ -10 



•3 „ '5 



'2 „ '5 



J 



0-1 to -3 



•10 „ -15 



•5 „ -8 



•5 „ 1-0 



•1 to -2 



•3 „ -5 



•15 „ -2 



•8 „ 1-2 





•2 „ *4 



•5 „ '8 



•2 „ *5 







•4 „ 1-0 



•8 „ 2-0 



•5 „ '8 







1-0 „ 2-0 





It is here noticeable that the amounts of depreciation for 

 given increments in current-density observed with two opposed 



