so makes the
velocity of the flow of the gas uniform across the section of the
tube. The Rontgen rays to ionize the gas were produced by a bulb at
O, the bulb and coil being in a lead-covered box, with an aluminium
window through which the rays passed. Q is connected with one pole of
a battery of cells, P and the other pole of the battery are put to
earth. The changes in the potential of T are due to ions giving up
their charges to it. With a given velocity of air-blast the potential
of T was found not to change unless the difference of potential
between P and Q exceeded a critical value. The field corresponding to
this critical value thus made the ions move with the known velocity of
the blast.

[Illustration: FIG. 8.]

[Illustration: FIG. 9.]

Another method which has been employed by Rutherford and McClelland is
based on the action of an electric field in destroying the
conductivity of gas streaming through it. Suppose that BAB, DCD (fig.
9) are a system of parallel plates boxed in so that a stream of gas,
after flowing between BB, passes between DD without any loss of gas in
the interval. Suppose the plates DD are insulated, and connected with
one pair of quadrants of an electrometer, by charging up C to a
sufficiently high potential we can drive all the positive ions which
enter the system DCD against the plates D; this will cause a deflexion
of the electrometer, which in one second will be proportional to the
number of positive ions which have entered the system in that time. If
we charge A up to a high potential, B being put to earth, we shall
find that the deflexion of the electrometer connected with DD is less
than it was when A and B were at the same potential, because some of
the positive ions in their passage through BAB are driven against the
plates B. If u is the velocity along the lines of force in the uniform
electric field between A and B, and t the time it takes for the gas to
pass through BAB, then all the positive ions within a distance ut of
the plates B will be driven up against these plates, and thus if the
positive ions are equally distributed through the gas, the number of
positive ions which emerge from the system when the electric field is
on will bear to the number which emerge when the field is off the
ratio of 1 - ut/l to unity, where l is the distance between A and B.
This ratio is equal to the ratio of the deflex