| Electrical Discharge in Helium and Neon. 99 
‘similar to those in fig. 2 obtained. The bend occurred at about 
6 mm. pressure, and the minimum spark potential was 150 volts 
at 15 mm. pressure. The cathode fall was 85 volts. Mey found 
78°5 for helium (Verh. deut. physikal. Ges. 1903, 5, p. 72). 
_ 2. A similar tube with an aluminium disk upon which were 
a few drops of sodium potassium alloy as one electrode, and a 
elean aluminium wire normal to it and 1°5 cm. away as the other. 
‘The first neon introduced was not perfectly pure and the surface 
of the alloy tarnished. When in this condition at 20 mm. pressure 
‘a 300 volt alternating current was completely rectified. After 
‘shaking to clean the surface of the alloy, and refilling with pure 
neon at 35 mm. the spark potential with plane as cathode was 
(145 and the cathode fall 85. When the tube was filled for the 
first time, the minimum spark potential with the wire as cathode 
was 155 and the cathode fall 130, on the second filling the latter 
had risen to 142. These figures are much lower than those 
obtained in the original spark potential tube, and it seems quite 
possible that they are due to the freshness of the electrode which 
had never been used before and was probably coated with a film 
of oxide, or was evolving hydrogen. The gas in the present case 
would also, if possible, be purer. . 
3. A tube 22 cm. long and 2°5 cm. wide, with an aluminium 
strip running the whole length as cathode. A sodium rivet was 
fixed in this, but by the time the tube was filled it was slightly 
tarnished. The anode was a wire in the form of an inverted U 
passing along each side of the strip. This tube was filled with 
eon at varying pressures, and showed a minimum spark potential 
of 209 at 5 mm. It was then refilled at this pressure and used 
for experiments which will be described later. 
4. A tube 14 cm. long, and 2 cm. wide with two copper sheets 
in the same plane along its axis and their ends 3 mm. apart as 
electrodes. Minimum spark potential 270 volts at 65 mm. 
pressure of neon, and cathode fall 221 volts. 
5. A tube the same size as the above with two square carbon 
rods 5mm. apart at the ends as electrodes. Constant results 
could not be obtained even after frequent refilling and passing 
heavy currents. The lowest values obtained for the spark potential 
and cathode fall were 360 and 217 volts respectively. 
6. A tube of the same size with an aluminium plate 
covered with sheets of very slightly tarnished calcium as cathode. 
U shaped anode. Minimum spark potential 200, cathode fall 
150 volts. 
7. Similar tube with spirals of magnesium wire as electrodes. 
Results very inconsistent. A value of 229 for minimum spark 
potential was obtained, and one of 150 for cathode fall. 
7—2 
