388 Prof. Skinner on the Relation of Electrode Fall 
In this paper are presented the results of experiments made 
to study this relation. The hypothesis is: The fall of the 
potential at the cathode in a given gas decreases as the electro- 
positive value of the metal increases; the anode fall, on the 
other hand, decreases as the electronegative value of the metal 
increases. 
Plan and Preparation. 
The metals tested were mainly chosen with reference to 
their position in the contact-potential series. They were: 
platinum, silver, gold, copper, steel, nickel, bismuth, antimony, 
tin, lead, cadmium, zine, aluminium, and magnesium. The 
gases used were: hydrogen, on account of the great difference 
it gives between the cathode-fall of platinum and aluminium; 
nitrogen, because of its chemical neutrality; and oxygen, 
because of the vital relation, in theory, between the contact 
potential of the metal and its affinity for oxygen. 
The plan of procedure was to measure in the given gas the 
electrode fall of all the metals under, so far as possible, exactly 
the same conditions. It was found impossible to obtain the 
fall sufficiently definite by testing first one metal, dismantling 
the discharge-tube, then introducing a new one and repeating 
the measurement. The fluctuation in value given by the 
same metal under successive tests of this kind was generally 
great enough to prevent a satisfactory comparison of the 
whole series. Neither were satisfactory results obtained by 
mounting the several metals in similar discharge-tubes and 
setting these up in open connexion with each other. A 
discharge passing through one tube produced often a much 
greater variation of the electrode fall in the neighbouring 
tubes than in the active one. 
The scheme finally adopted, which proved wholly satis- 
factory, was to bring, in rapid succession, the metals to be 
tested into the same position in one discharge-tube and 
measure the electrode fall under invariable conditions, the 
results being discarded if a disturbing change took place in 
the gas during the series of observations. 
Fig. 1 represents a vertical section of the discharge-tube 
ending below in a larger cylindrical chamber (axis vertical), 
the whole of glass construction. Ten electrodes E (circular 
disks 15 mms. in diameter) were attached to the face of a 
circular glass plate at equal intervals by hard rubber caps K 
(see also plan in fig. 2). This glass plate, mounted in the 
cylindrical chamber, could be rotated about its axis by the 
key introduced through the ground joint J, and by this means 
any electrode brought into position at the lower end of the 
