|
| | Platinum Electrodes. |Aluminium | required to |
| | |Electrodes.|maintain a Spark.|
+---------+-----------+---------+-------+-----------+-----------------+
| | Warburg. |Capstick.|Strutt.| Warburg. | Strutt. |
+---------+-----------+---------+-------+-----------+-----------------+
|Air | 340-350 | .. | .. | .. | 341 |
|H2 | about 300 | 298 | .. | 168 | 302-308 |
|O2 | .. | 369 | .. | .. | .. |
|N2 |230 if free| 232 | .. | 207 | 251 |
| |from oxygen| | | | |
|Hg vapour| 340 | .. | .. | .. | .. |
|Helium | .. | .. | 226 | .. | 261-326 |
|H2O | .. | 469 | .. | .. | .. |
|NH3 | .. | 582 | .. | .. | .. |
+---------+-----------+---------+-------+-----------+-----------------+

Thus in the cases in which the measurements could be made with the
greatest accuracy the agreement between the cathode fall and the minimum
potential difference is very close. The cathode fall depends on the
material of which the terminals are made, as is shown by the following
table due to Mey (_Verh. deutsch. physik. Gesell._, 1903, 5, p. 72).

+------+---------------------------------------------+
| Gas. | Electrode. |
+------+---+---+---+---+---+---+---+---+---+-----+---+
| | Pt| Hg| Ag| Cu| Fe| Zn| Al| Mg| Na| Na-K| K |
+------+---+---+---+---+---+---+---+---+---+-----+---+
|O2 |369| ..| ..| ..| ..| ..| ..| ..| ..| .. | ..|
|H2 |300| ..|295|280|230|213|190|168|185|169 |172|
|N2 |232|226| ..| ..| ..| ..| ..|207|178|125 |170|
|He |226| ..| ..| ..| ..| ..| ..| ..| 80| 78.5| 69|
|Argon |167| ..| ..| ..| ..| ..|100| ..| ..| .. | ..|
+------+---+---+---+---+---+---+---+---+---+-----+---+

The dependence of the minimum potential required to produce a spark upon
the metal of which the cathode is made has not been clearly established,
some observers being unable to detect any difference between the
potential required to spark between electrodes of aluminium and those of
brass, while others thought they had detected such a difference. It is
only with sp