52 
PROFESSOR E. RUTHERFORD AKD MR. R. K. McCLUNG 
Least Potential, required to qjroduce a SqKirJc in Air. 
The ions in the “electric ^vind” in air have been shown to move with the same 
velocity as the ions produced by X rays. It is probable that the passage of a spark 
between two electrodes is heralded l)y tlie production of ions in the gas, and that 
these ions are of the same kind as the ions in air produced by X rays. 
Let V be the difference of potential between two electrodes in air, one electrode 
being connected to earth. Suppose a pair of ions to be produced and to travel to 
the electrodes. A cpiantity of energy 3'8 X erg is absorbed in their produc¬ 
tion, while the energy of the electric S 3 ^stem is diminished by an amount Ve. The 
energy recpiired to produce the ions must be derived from the electric energ}^ of the 
system. In order for an ion to be produced consistent with the conservation of 
energy, we must have Y of such value that Ye is greater than the energy required to 
produce a pair of ions. 
;j-8 X 10~“ 
Y > TV-electrostatic unit. 
b'o X 10 
Y > 175 volts. 
Now Peace (J. J. Thomson, ‘Pecent Pesearches,’ p. 89) has shown that it is 
impossi1)le to produce a spark in air Irelow alrout 300 volts, however close the 
electrodes are together. This is a somewhat greater value than the one found 
above, Irut is of the same order. Strutt"”' has recently showui that the minimum 
potential difference for the passage of a s]jark in pure nitrogen is about 251 volts. 
As most of the ions in air are probably produced from the nitrogen molecides, this 
value makes the agreement still closer. The results obtained would indicate that it 
would be impossible to produce an ion, and therefore an electric spark, below 175 volts. 
If the energy required to produce an ion w’ere the same at all pressures, the minimum 
sparking potential according to the alcove theory would be unaltered. This is borne 
out 1jy Peace’s results {Joe. cit., }). 8G), wdiere it is showui that the minimum 
potential difference for a spark lietween spherical electrodes '001 centim. apart is 
approximately the same for pressures from 300 to 50 millims. of mercuiAu The 
minimum potential rises below 50 millims., indicating that the energy required to 
produce an ion may possilily increase below that pressure. 
This theory wmuld suggest that the minimum potential required to produce a spark 
conversely might be used as a means of determining the energy to produce an ion. 
The jihenomenon, liowever, is moi'e conqdex than this wmuld indicate. The minimum 
sjiarking potential is to a small extent influenced by the metal used for the electrodes 
and also by the gas, and moreover it would leave unexplained the remarkable fact 
that wdien the electrodes are a small distance apart the spark does not follow the 
shortest path [loc. cit.) between them. 
* ‘ Phil. Trans.,’ A, 1900. 
