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$6 RA YS OF POSITIVE ELECTRICITY

FOR MEASURING THE NUMBER OF
POSITIVELY ELECTRIFIED PARTICLES.

Though the photographic plate furnishes an excellent means
of detecting the existence of positively charged particles of
different kinds it Is not suitable for comparing the number of
particles present in a bundle of positive rays. For. though
the Intensity of the lines on the photograph will vary with the
number of particles, this number will not be the only factor In
the expression for the intensity. As an example, consider the
due (i) to very light particles like the atoms of hydrogen,
and (2) to very, heavy ones like the atoms of mercury. If
these particles have acquired the same amount of energy in the
electric field before entering the cathode, the hydrogen atoms
will have a velocity about fourteen times that of the mercury
ones: they might therefore be expected to penetrate further
into the ilm on the plate and produce a greater photographic
effect than the mercury ones. If this expectation Is realized,
and we shall see that It is, It Is evident that the photographic
effect cannot be taken as a measure of the number of positively
electrified particles.

A method which does give metrical results is founded on
the following principle. Suppose that we replace the photo-
graphic plate in the preceding method by a metal plate in
which there is a movable parabolic slit, then when this slit is
moved Into such a position that It coincides with one of the
parabolas on the photographic plate, positively electrified par-
ticles would pass through the slit; if these particles are caught
and their total charge measured we shall have a measure
of the number of positively electrified particles. Thus if the
slit were gradually moved up the plate there would be no charge
coining through it, unless it coincided In position with one of
the parabolas. As one parabola after another was passed, posi-



	$6 RA YS OF POSITIVE ELECTRICITY FOR MEASURING THE NUMBER OF POSITIVELY ELECTRIFIED PARTICLES. Though the photographic plate furnishes an excellent means of detecting the existence of positively charged particles of different kinds it Is not suitable for comparing the number of particles present in a bundle of positive rays. For. though the Intensity of the lines on the photograph will vary with the number of particles, this number will not be the only factor In the expression for the intensity. As an example, consider the due (i) to very light particles like the atoms of hydrogen, and (2) to very, heavy ones like the atoms of mercury. If these particles have acquired the same amount of energy in the electric field before entering the cathode, the hydrogen atoms will have a velocity about fourteen times that of the mercury ones: they might therefore be expected to penetrate further into the ilm on the plate and produce a greater photographic effect than the mercury ones. If this expectation Is realized, and we shall see that It is, It Is evident that the photographic effect cannot be taken as a measure of the number of positively electrified particles. A method which does give metrical results is founded on the following principle. Suppose that we replace the photo- graphic plate in the preceding method by a metal plate in which there is a movable parabolic slit, then when this slit is moved Into such a position that It coincides with one of the parabolas on the photographic plate, positively electrified par- ticles would pass through the slit; if these particles are caught and their total charge measured we shall have a measure of the number of positively electrified particles. Thus if the slit were gradually moved up the plate there would be no charge coining through it, unless it coincided In position with one of the parabolas. As one parabola after another was passed, posi-