Dimension* of the Cathode Images, 615 



cathode covered by the glow. Corresponding results were 

 obtained with plates. 



Thus, so long as the cathode is not fully covered and the 

 pressure is maintained constant, the current-density remains 

 constant. 



2. Relation connecting the area covered by the glow witli the 

 pressure when the current is constant. The measurements 

 were carried out with widely varying currents, and in the 

 case of pure aluminium cathodes in nitrogen led to the result 

 that with a constant current the cathode area covered by the 

 glow is inversely proportional to the pressure, so that 



2irrlp— constant, (2) 



where p is the pressure. 



3. Relation connecting current-density and pressure. From 

 the results expressed by (1) and (2) it follows further that 

 for aluminium cathodes in nitrogen the following proposition 

 holds :— 



The current-density at the cathode is directly proportional to 

 the pressure, i. e., 



— constant (3) 



zirrlp 



■i. Connexion between current-density and pressure with 

 different electrode materials. — The materials investigated were 

 aluminium, platinum, and graphite. In the case of platinum, 

 the relation found for aluminium and expressed by (3) was 

 found to hold good, viz., that the current-density is directly 

 proportional to the pressure, the only difference being in the 

 value of the constant, which is 0*47 for aluminium, and 0*33 

 for platinum, if in equation (3) the current is expressed in 

 milliampereSj the radius?' and length / in cms., and p in mm. 

 of mercury. Xo exact measurements were possible in the 

 case of graphite by reason of the rapid disintegration*. 



'). Connexion between current-density and pressure in 



different oases. — Preliminary observations showed that the 



relation (3) 



/ 



= constant 

 'lirrlp 



only holds for aluminium in nitrogen, but that for any given 

 metal in a given gas we must write 



-=— = constant, (4^ 



■lirrlp" v ' 



bo that tli'' current-density varies as the n\\\ power of the 



* Mr. WilsOD finds the .-ame value of 04 lor aluminium and platinum. 



