el it ionized the gas inside and so greatly
increased its conductivity; thus, if the gas were kept exposed to an
electric field, the current through the gas would suddenly increase
when an [alpha] particle passed into the vessel. Although each [alpha]
particle produces about thirty thousand ions, this is hardly large
enough to produce the conductivity appreciable without the use of very
delicate apparatus; to increase the conductivity Rutherford took
advantage of the fact that ions, especially negative ones, when
exposed to a strong electric field, produce other ions by collision
against the molecules of the gas through which they are moving. By
suitably choosing the electric field and the pressure in the
ionization chamber, the 30,000 ions produced by each [alpha] particle
can be multiplied to such an extent that an appreciable current passes
through the ionization chamber on the arrival of each [alpha]
particle. An electrometer placed in series with this vessel will show
by its deflection when an [alpha] particle enters the chamber, and by
counting the number of deflections per minute we can determine the
number of [alpha] particles given out by the radium in that time.
Another method of counting this number is to let the particles fall on
a phosphorescent screen, and count the number of scintillations on the
screen in a certain time. Rutherford has shown that these two methods
give concordant results.

The charge of positive electricity given out by the radium was
measured by catching the [alpha] particles in a Faraday cylinder
placed in a very highly exhausted vessel, and measuring the charge per
minute received by this cylinder. In this way Rutherford showed that
the charge on the [alpha] particle was 9.4 X 10^-10 electrostatic
units. Now e/m for the [alpha] particle = 5 X 10^3, and there is
evidence that the [alpha] particle is a charged atom of helium; since
the atomic weight of helium is 4 and e/m for hydrogen is 10^4, it
follows that the charge on the helium atom is twice that on the
hydrogen, so that the charge on the hydrogen atom is 4.7 X 10^-10
electrostatic units.

_Calculation of the Mass of the Ions at Low Pressures._--Although at
ordinary pressures the ion seems to have a very complex structure and to
be the aggregate of many molecules, yet we have evidence that at very
low pressures the structure of the ion, and especially of the negative