The difference of potential between the cathode and the negative glow is called the "cathode potential fall" and is found to be constant for wide variations in the pressure of the gas and the current passing through. It increases, however, considerably when the current through the gas exceeds a certain critical value, depending among other things on the size of the cathode. This cathode fall of potential is shown by experiment to be very approximately equal to the minimum potential difference. The following table contains a comparison of the measurements of the cathode fall of potentials in various gases made by Warburg (_Wied. Ann._, 1887, 31, p. 545, and 1890, 40, p. 1), Capstick (_Proc. Roy. Society_, 1898, 63, p. 356), and Strutt (_Phil. Trans._, 1900, 193, p. 377), and the measurements by Strutt of the smallest difference of potential which will maintain a spark through these gases. +---------+-----------------------------------------+-----------------+ | | Cathode fall in Volts. |Least potential | | Gas. +-----------------------------+-----------+ difference | | | 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).