58 RA FS OF POSITIVE ELECTRICITY

Immediately behind the slit there is an insulated long, narrow
vessel placed so that every particle passing through the
slit falls into this vessel. This vessel is connected with a
Wilson tilted electroscope by which the charge it recei\7es can
be measured.

From the front face of the box a portion was cut away,
the opening closed by a willemite screen. The positive
rays could be deflected on to this screen and the brightness
of the fluorescence observed; in this way one can make sure
that the tube is in the proper state for giving positive rays
before attempting to make the measurements.

The impact on the face of the box of the rays which do
not pass through the slit gives rise to the emission of slowly
moving cathode rays; if precautions are not taken these diffuse
through the slit, enter the Faraday cylinder, and confuse the
measurements. This diffusion can be avoided by placing a
small permanent magnet near the slit The force due to this is
strong enough to deflect the more mobile cathode rays without
producing any appreciable effect on the positively charged
atoms. The pressure of the gas between this box and the
cathode should be made as small as possible: the best way of
reducing the pressure is to absorb the gas by means of charcoal
cooled with liquid air. This method will not produce a good
vacuum when the gas in the tube is helium; with hydrogen,
toos the vacuum is not so good as for heavier gases, for then the
pressure can by this means easily be reduced to 3/1000 of a
millimeter.

The method of observing with this apparatus is as follows:
The positive rays are deflected by a constant electric field of
such a magnitude that the heads of the parabolas are in line
with one end of the slit The magnetic field is then increased
by small increments and the deflection of the Wilson electro-
scope in ten seconds measured, Unless a parabola comes on though cathode rays may produce some charged atomshe liberation of all the cor-