Gesell._, 1907, 9, p. 69), and the
negative ions produced when ultra-violet light falls on a metal plate by
Rutherford (_Proc. Camb. Phil. Soc._ 9, p. 401). H. A. Wilson (_Phil.
Trans._ 192, p. 4O9), Marx (_Ann. de Phys._ 11, p. 765), Moreau (_Journ.
de Phys._ 4, 11, p. 558; _Ann. Chim. Phys._ 7, 30, p. 5) and Gold
(_Proc. Roy. Soc._ 79, p. 43) have investigated the velocities of ions
produced by putting various salts into flames; McClelland (_Phil. Mag._
46, p. 29) the velocity of the ions in gases sucked from the
neighbourhood of flames and arcs; Townsend (_Proc. Camb. Phil. Soc._ 9,
p. 345) and Bloch (_loc. cit._) the velocity of ions produced by
chemical reaction; and Chattock (_Phil. Mag._ [5], 48, p. 401) the
velocity of the ions produced when electricity escapes from a sharp
needle point into a gas.

Several methods have been employed to determine these velocities. The
one most frequently employed is to find the electromotive intensity
required to force an ion against the stream of gas moving with a known
velocity parallel to the lines of electric force. Thus, of two
perforated plane electrodes vertically over each other, suppose the
lower to be positively, the upper negatively electrified, and suppose
that the gas is streaming vertically downwards with the velocity V; then
unless the upward velocity of the positive ion is greater than V, no
positive electricity will reach the upper plate. If we increase the
strength of the field between the plates, and hence the upward velocity
of the positive ion, until the positive ions just begin to reach the
upper plate, we know that with this strength of field the velocity of
the positive ion is equal to V. By this method, which has been used by
Rutherford, Zeleny and H. A. Wilson, the velocity of ions in fields of
various strengths has been determined.

The arrangement used by Zeleny is represented in fig. 8. P and Q are
square brass plates. They are bored through their centres, and to the
openings the tubes R and S are attached, the space between the plates
being covered in so as to form a closed box. K is a piece of wire
gauze completely covering the opening in Q; T is an insulated piece of
wire gauze nearly but not quite filling the opening in the plate P,
and connected with one pair of quadrants of an electrometer E. A plug
of glass wool G filters out the dust from a stream of gas which enters
the vessel by the tube D and leaves it by F; this plug al