12 Newbery, Recent Work on Ovcrvoltage. 



metals show one, and only one, definite \alue in a given elec- 

 trolyte, with small variations above and below this value. 

 Thallium, chromium and rhodium in acid electrolyte show this 

 property well. Others show two such values with rapid changes 

 from one to the other. Thus, iridium, when first used, shows 

 low values approaching zero, but on subjection to the highest 

 current density employed, a rapid rise to about 0.18 volt occurs, 

 and this value persists throughout the range of current density 

 used, although a small fall is produced at the highest current 

 densities. In alkali, again, this metal shows two very distinct 

 values, with a sudden change from the lower to the higher 

 when the current density is raised from 100 to 200 milliamperes 

 per sq. cm., and a similar sudden fall when the current density 

 again reaches a low value. By maintaining the current density 

 at about 200 milliamperes per sq. cm., the overvoltage rises to 

 0.7 volt or higher. Thus iridium shows four quite definite values 

 for the hydrogen overvoltage. 



By proceeding in this way with all the electrodes used, 

 certain more or less definite average values for the cathodic 

 hydrogen overvoltages may be assigned to all the Inetals. 



•When these values are placed with the metals in the periodic 

 table a new law at once becomes evident — Ele?nents in the same 

 group of the periodic systei7i show the same overvoltage. 



The following table illustrates this law : — 



Cathodic Overvoltage in the Periodic System. 



Carbor 

 — 0.44 



0.70 



Mag- Alu- 



Sodium nesium minium 

 3 0.34? 0.70 0.50 



<0.I0 



Copper Zinc 



0.34 0.72 

 0.60 



Chro- Manga- 



mium nese Iron Nickel Cobalt 



0.38 0.30 0.18 0.18 



42 0.48 0.60 0.25 0.33 0.24 0.30 0.25 0.67 



Moly- 



denum Rhodium Palladium 



032 — — 0.19 0.05 



002 0.34 0.65 



Anti- 

 Silver Cadmium Tin mony 

 0.30 0.66 ^— 0.45 0.42 

 0.06 0.50 0.66 0.66 



Tantalum Tungsten Iridium Platinum 



— 0.41 0.30 — — 0.18 0.04 01 8 0.06 



0.50 0.45 0.67 0.45 0.65 



Gold Mercury Thallium Lead Bismuth 

 II 0.32 0.70 0.52 0.46 43 



0.50 0.00 0.72 e.52 



