Manchester Memoirs, Vol. lx. (1916), No. 11. 17 



voltage of lead, while i*o % of the same colloid had no 

 effect on the overvoltage of zinc, but raised that of lead 

 by o 1 volt. Again 002 % gum arabic lowered the over- 

 voltage of lead by O'Oi volt, while 4-5 % had no effect. 



Poisons such as strychnine, brucine, nicotine, etc.. 

 sometimes raise and sometimes lower overvoltages. 



It is evident, therefore, that no definite statement can 

 be made as to the effects of colloids or poisons in general. 



One point is, however, of interest here. The over- 

 voltage of platinum is hardly affected by the presence of 

 °'°5% gelatine at low current densities, but at higher cur- 

 rent densities the overvoltage is kept practically constant, 

 while without the colloid the overvoltage changes con- 

 siderably. The colloid here appears to exert a stabilising 

 effect, preventing the fall of overvoltage at high current 

 densities — a phenomenon intimately connected with the 

 concurrent lowering of the ionisation of the liberated gas. 



The following table illustrates this : — 





Overv 



oltage. 



Current density per 







sq. cm. 









With colloid. 



Without colloid. 



20 milliamperes 



008 volt 



o*o6 volt 



50 



°'°9 55 



0-06 „ 



TOO 



O'lO ,, 



007 „ 



200 



O'lO „ 



0-08 „ 



500 „ 



O'lO ,, 



o'°5 » 



1,000 ,, 



O'lO ,, 



0-03 „ 



2,000 „ 



0*09 ,, 



-0*03 „ 



