182 Medway — Material and Shape of the Rotating Cathode. 



deposit is that the edge of the disc, being nearer the anode, 

 receives more current and larger deposit of copper than the 

 central portions, with the consequence that the deposit upon 

 the edge, built out and fragile, tends to break off under the 

 rapid rotation of the disc. To remedy this defect, I have 

 used for an anode a strip of platinum extending across the cell 

 under and parallel to the rotating disc, so that the current may 

 be equalized all over the disc. With the apparatus arranged in 

 this manner, the following very good results were obtained in 

 the precipitation of copper. 



Deposition upon the Platinum Disc. 





Copper 



Copper 













taken. 



found. 



Error. 



Current. 



N. D. 



Time 





grm. 



grm. 



grm. 



Amp. 



100. 



min. 



(1) 



0-0670 



0-0672 



+ 0-0002 



2- 



12 



15 



(2) 



0-0670 



0-0668 



— 0-0002 



2* 



• 12 



15 



(3) 



0-0670 



0*0666 



— 0-0004 



2- 



12 



15 



(*) 



0-0670 



0-0671 



+ 0-0001 



2-5 



15 



15 



(5) 



0-0670 



0-0670 



±0-0000 



2- 



12 



15 



The attempt to substitute aluminum for platinum, though 

 the former metal has been recommended by Hough,* as 

 material to receive the deposit, was not successful, owing prob- 

 ably to the film of aluminum oxide always present. Various 

 measures were taken to remove this film — e. g., the aluminum 

 was treated with hydrochloric acid and quickly transferred to 

 the solution, a few drops of hydrofluoric acid added before the 

 current was passed — but all to no avail, the copper falling off 

 as fast as deposited. 



From my experience, it seems that the disc is inferior to the 

 crucible for use as a rotating cathode. Not only does the disc 

 fail to hold the deposited copper as well as the crucible, but 

 there are difficulties of manipulation which render the drying 

 and weighing of the deposit upon the disc comparatively 

 inexact. 



* Jour. Am. Chein. Soc, xx, 302. 



