282
THE
RELATIVITY PRINCIPLE
Fig.
1
(actual size)
[53]
His apparatus,
whose main
parts
are
depicted
in their actual size in
Fig.
1,
consisted essentially
of
a
lightproof
brass
casing
H
placed
within
an
evacuated
glass
vessel, with
a
radium
granule placed
in
a
small
well
0
in
the floor
A
of
the
casing. The
/?-rays emanating
from
the
radium
pass
through
the
gap
between
two
condenser
plates
P1
and
P2,
cross
the
diaphragm
D,
whose
diameter is 0.2
mm,
and
then fall
on
the
photographic
plate.
The
rays
were
deflected
both
by an
electric field
formed between
the condenser
plates
P1
and
P2
and
by a
magnetic
field
of the
same
direction
(produced
by
a
large
permanent
magnet), perpendicular
to
that direction
so
that
rays
of
the
same
velocity marked
a
point,
and
the
aggregate
of
the particles
of
different velocities marked
a curve on
the plate.