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IONIZATION IN ATMOSPHERIC AIR. 
By J. A. McCLELLAND, M.A., 
Professor of Wxperimental Physics, University College, Dublin. 
[Read Junr 16, 1903.] 
1.—JIntroduction. 
In the last ten years ve have become familiar with many methods of making a 
gas a conductor of electricity. A gas becomes a conductor when subjected to the 
action of Riéntgen rays, or cathode rays, or to the radiation given off by 
uranium, thorium, radium, or other radio-active substance; gas drawn from an 
are or a flame, or from the neighbourhood of an incandescent metal, is a conductor ; 
so also is gas newly generated by an electrolytic cell, and other cases of conduc- 
tivity in gases might be mentioned. In all these cases it has been shown that the 
conductivity of the gas is due to the presence in it of positively and negatively 
charged particles, which travel through the gas when exposed to the action of 
electric force, and this stream of charged particles—positive going in one direction, 
negative in the opposite—constitutes an electric current. 
Further, it has been shown that these positively and negatively charged 
particles result from the breaking up of the atom into two parts, having equal but 
opposite charges. 
This production of positively and negatively charged particles by the breaking 
up of some of the atoms is called the ionization of the gas. When a gas is ionized, 
even a very weak electric force sets the ions in motion, and produces an electric 
current; in the normal state of a gas, when not ionized, we can only force an 
electric discharge through it by using very strong electric force, sufficiently 
strong to cause directly or indirectly the splitting up of the atom. 
Up to a short time ago we looked upon a gas before it had been exposed to any 
of the ionizing agencies mentioned above, as absolutely a non-conductor; but in 
1900, C. 'T. R. Wilson* in Cambridge, and Geitelt in Germany, showed that there 
is always a slight ionization in atmospheric air. 
* Proc, Camb. Phil. Soc. } Physikalische Zeitschrift, 2, Jahrgang, No. 8, pp. 116-119. 
TRANS, ROY. DUB, SOC., N.S., VOL. VIII., PART IV, M 
