532 Prof. K. T. Compton on 



hydrogen and atomic iodine, produced by dissociation within 

 an incandescent tungsten tube, are now in progress. 



The two characteristics of monatomic gases, which probably 

 account for the relative ease of production of low voltage 

 arcs in them, are elasticity of electron impacts and ability of 

 any atom to absorb the resonance radiation from neighbouring 

 atoms. The former characteristic results in the gaining of 

 sufficient energy by every electron to produce either radiation 

 or ionization at an impact, and also in greatly increasing 

 the number of impacts made by an electron in its path 

 between the electrodes. Tbe latter characteristic permits 

 the radiant energy, liberated by each electron impact which 

 results in radiation, to be passed on from atom to atom, and 

 thus multiplies the fraction of atoms which are in the 

 "abnormal" or partially ionized condition. In multiatomic 

 gases the undissociated molecules do not appear to be capable 

 of absorbing tlie radiation which is produced by electron 

 impacts and which is believed to be characteristic of the 

 dissociated atoms instead of the molecules. 



In a previous paper * the writer has summarized some 

 evidence that ionization in low voltage arcs cannot be due 

 to effects of single impacts, but must be the cumulative 

 effect of two or more impacts. Additional evidence may be 

 suggested along the following lines. 



Consider parallel electrodes distant d apart in a gas at 

 pressure p millimetres and with a difference of potential V, 

 giving an average electric intensity X = V/d. Let V; and 

 V r be the minimum ionizing and radiating potentials of the 

 gas and let N be the average number of collisions made by 

 an electron per centimetre path. Its mean free path Z = l/N. 

 If the potential difference V is made equal to or slightly 

 greater than V h what is the probability P that an electron 

 may gain a speed sufficient to ionize a normal atom by 

 impact ? In a monatomic gas every electron will attain a 

 speed corresponding to V r despite possible collisions. In 

 passing from the point of potential V r to that of potential V t -, 

 it is liable to expend its energy in the production of radiation 

 at any impact. If one of these intervening impacts happens 

 to be elastic, the electron is deflected through such an 

 increased path that additional impacts become probable, each 

 with an additional chance for loss of energy by production 

 of radiation. Taking account of this and of the average 

 direction of motion of an electron in the region between Y r - 

 and Vi, it is found that the probability of getting through 



* Phys. Rev. xv. p. 476 (1920). 



