Polarization at a Metallic Anode. 655 



with sealing-wax. Two anodes and tubes, (1) & (2), of 

 different areas were used. 



When the current passing through the cell is produced by 

 an applied E.M.F. of only a few volts, it rapidly falls off, 

 due to the back-E.M.F. of polarization set up. In most 

 cases it was desirable to keep the current constant, so as to 

 simplify the arithmetical calculations from equation (24) : to 

 effect this a battery of 64 secondary cells, applied to the 

 polarizable cell through a resistance of 60,000 + 500 ohms, 

 was used to produce the current. The current was measured 

 by observing on a direct-reading potentiometer the potential- 

 difference between the ends of the 500 ohms. The potential- 

 difference between the anode A and the free electrode B of 

 rhe cell was measured on the same potentiometer in terms of 

 the mean of two Clark cells kept in a water-bath. Care was 

 of course taken to avoid affecting the polarization by allowing 

 any appreciable current to pass between A and B in taking 

 the readings. To avoid this, a resistance of 10,000 ohms was 

 kept permanently in the galvanometer circuit, and in addition 

 only momentary contacts of the potentiometer key were made. 



During the period of recovery of the cell, when the 

 polarizing current is no longer on, the true polarization V 

 many be thus measured direct. But while the current is 

 being applied, the observed potential-differences between the 

 anode A and the free electrode B give the quantity V + CR AB , 

 where C is the current, and R AB the resistance between these 

 two points ; and to obtain the true polarization a knowledge 

 of this resistance is necessary. I found it was impossible to 

 measure this accurately on the Wheatstone's bridge, using a 

 secohm meter to reverse the battery and galvanometer 

 connexions simultaneously. Even at the highest speed at which 

 cohmmeter could be rotated, the resistance as measured 

 altered from 50 to 57 ohms when the cell AB was reversed on 

 the bridge. This is due to the fact that the cell polarizes much 

 more rapidly when the current passes through it in the direction 

 from B to A than when from A to B (the electrode B being 

 larger than A). As an accurate determination of the resistance 

 was impracticable, in the following tables the values of the 

 total potential-differences are simply tabulated as observed in 

 the columns headed (V — CK AI ,j ob8 , and their agreement with 

 the theory tested as follows : — During the period in which a 

 constant polarising current is being applied, the excesses of 

 the value- of ( V + ('ll A ,,j ob8 . over the calculated values of V 

 -hould represent the quantity CR U! and remain constant. 

 From the mean value for each experiment values of R AB may 

 bseqnently calculated and compared with each other. 

 2X2 



