°
_
8—5.
MOBÏLÏTŸ
163
.
grid, which they reach after the lapse of a denite time, depending
on their mobilities. The same
potential used to produce the pulses of ions is used to prevent their continued passage
beyond the second grid to a collecting plate and electrometer,
except during the time when the latter attracts rather than repels
the ions.
This occurs when the period of the alternations is equal
to the time for the ions to travel from the rst grid to the second,
.or some integral multiple thereof. É
‘
The mobility of “ old
”
ions in dry air is 1.6 for the positive
and 2.18 for the negative, as measured by Loeb and his collaborators.
For most gasesthe positive ions move more slowly than
the negative. However, for freshly formed positive ions in air,
the mdbility is just as great as for the negative. The mobility of
ions is inversely proportional to the pressur’eover wide ranges and
is independent of the inertia of the ion, In certain gases such
as hydrogen, 'helium, nitrogen and mercury vapor, the electron
_
by ionization remains free. The mobility of electrons is
very great, ranging from several hundred to many thousand centi—
meters per second per volt,per Centimeter. In other gases (the
electro-negative) such as oxygen, chlorine and water vapor, the
electron
itself to‘ a
molecule and forms a
a«tive ion.
_
The ion [in general consists of the initially charged
__
posit—iVe
molécule or atom and, according to the nature .
of the surrounding gas/‘andfthe electrochemical afnity of those
gas molécules, may have one to three foreign» moleCules attached,
the conibih—a—tionsg being
such as '
'
the positive :»molecdlèÏ—3àii0nia ion,
molecule—methyl
amine; ion, positive
or, «the negatively charged .
_
.
.
moleculeltmust beemhas1led that the ‘iomf willone
1mpur1ty attachedandtak€anotherone {Which i’SÎ, more: strengl—y
attractedThusamman1areplaœssemeunknownconstituent in;
gateofsmt0twentÿmüïecules8
'