464 BELL SYSTEM TECHNICAL JOURNAL 



While self-sustaininj? discharges in gases may produce a vast variety 

 of identifiable ions, they are not suitable for revealing the process of 

 producing these ions. By bombarding a gas with electrons of known 

 speed, ions may be produced under very simple and intelligil)le con- 

 ditions. It is then found that in order to detach an electron from an 

 atom of a monatomic gas, a definite amount of energy, the ionizing- 

 energy of the gas, must be transferred to the atom. The ionizing- 

 energy is not unique; for most kinds of atoms there are several dis- 

 tinct quantities answering to the same definition. Nevertheless there 

 is one particular and outstanding value which is particularly known 

 as the ionizing-energy or ionizing-potential. It varies periodically 

 from element to element along the Periodic Table, and is therefore 

 ascribed to an outer electron of the atom; indeed it may be described 

 as the extraction-energy for the outermost or loosest electron. 



Of the other values of ionizing-energy for a given atom, some are 

 lower than the principal ionizing-potential. These, however, are 

 attributed to atoms in abnormal states. The others are greater 

 than the principal ionizing-potential; some of them are very much 

 greater and increase steadily from one element to the next along the 

 Periodic Table, and are therefore ascribed to deeper-lying electrons 

 and may be described as extraction-energies for inner electrons. 



The spontaneous ionization of radioactive substances is an entirely 

 irregular function of atomic number and is attributed, for this and 

 other reasons, to events occurring in the nuclei. 



At this point it is necessary to define some units. In most determi- 

 nations of ionizing-energies, a stream of electrons originally moving 

 with speeds thought negligibly small is accelerated by a potential- 

 rise and then projected into the gas under examination. Their kinetic 

 energies in ergs are thus given in terms of the voltage V of the poten- 

 tial-rise by the equation 



Kinetic Energy = eF/300 = 1.59110-'2 V. (1) 



It is customary to measure the kinetic energy of an electron by the 

 voltage-rise which gave it, or could have given it, that energy; which 

 is tantamount to employing a unit of energy equal to 1.591.10~^~ erg. 

 This unit may be called the equivalent volt. 



One equivalent i'o// = 1.591-10"^^ erg (2) 



The name, it must be admitted, is neither short nor elegant; at all 

 e\ents it is preferable to the slo\enly usage of speaking of an electron 

 as ha\-ing so many 'Solts of energy" (!) or a "speed of so many volts" 

 (!!) On the other hand, it seems quite unobjectionable to speak of an 



