392 ~~ Prof. Skinner on the Relation of Electrode Fali 
being kept in use longer than was absolutely necessary. This 
plan was followed throughout the investigation. 
Cathode Fall in Hydrogen.—The cathode-fall in hydrogen, 
as in nitrogen (I. ¢.), increases for small currents in a linear 
manner with the current-density; it also increases, for the 
same current-density, as the gas-pressure diminishes—this 
latter, however, not in the simple manner found tor nitrogen. 
Here also the rate of increase both with current-density and with 
decreasing gas-pressure is notably greater for those metals 
possessing the larger minimum, or normal, cathode-fall. In 
nitrogen the difference in fall between the metals is so small 
that this factor, though observable, was considered negligible. 
Observations in hydrogen are represented by curves in fig. 3. 
They show the relation between the electric current and the 
cathode-fall for platinum, zinc, and aluminium—each with the 
gas ata pressure 1°0, 1°72,and 3°0 millims.of mercury. At each 
pressure the observations were made for all three metals without 
changing the gas. Those at the ditferent pressures were also 
taken, in the sequence 1°72, 3:0, and 1:0 millims., without in 
the meantime repolishing the electrodes. The sequence of 
observations for each curve, indicated by the arrows, shows the 
magnitude of the change which took place with use. This 
was always an increase, generally more rapid with those 
metals possessing the greater fall. The curves end on the left 
at about the minimum current-density, at which point the 
area of the discharge begins to contract with a further 
reduction of the current, and the cathode-fall becomes con- 
stant, as shown by the curve Al 1:°72*. In addition to these, 
a set of observations were made at 2 mms. gas-pressure, com- 
paring platinum and gold as cathodes. The difference in 
cathode-fall of these remained 5 volts throughout a range 
from 0°6 m.a. to 3m.a. From these results it is to be con- 
cluded that the cathode series for hydrogen is the same 
whatever be the current-density and gas-pressure at which it 
be taken. 
The Cathode Fall in Oxygen.—This exhibits the same 
qualities as hydrogen, with the interesting distinction that 
while the minimum cathode-fall is of about the same mag- 
nitude, its increase with current-density, and with diminishing 
* While making these observations it was also noticed that the 
minimum current-density is smaller for the metal possessing the larger 
cathode-fall; for example, at 3:0 mms. pressure the discharge area for 
aluminium began to contract when the current was reduced below 
13 m.a.; for zinc, below 0°8 m.a.; andfor platinum, below 0°65 m.a. At 
lower pressures this transition-point becomes unobservable. 
