196
THEORY OF LIGHT
PRODUCTION
However,
this
assumption
involves
yet
a
second
one,
because it
contra-
dicts the
theoretical basis
from which equation
(3)
is
developed. For if the
energy
of
a
resonator
can
only
change
in
jumps,
then the
mean
energy
of
a
resonator
in
a
radiation
space
cannot be
obtained
from
the usual
theory
of
electricity, because the latter
does not
recognize
distinguished
energy
values
of
a
resonator.
Thus,
the
following assumption
underlies Planck's
theory:
Although
Maxwell's
theory
is
not applicable to
elementary resonators,
nevertheless
the
mean
energy
of
an
elementary
resonator
in
a
radiation
space
[17]
is
equal
to
the
energy
calculated
by means
of
Maxwell's
theory
of electricity.
This
proposition
would be
immediately
plausible
if,
in all
those
parts
of
the
spectrum
that
are
relevant for
observation,
e
=
(R/N)ßv were
small
compared
with the
mean
energy
Ev
of
a
resonator; however,
this is
not at
all
the
case,
for within the
range
of
validity
of
Wien's
radiation formula,
eBv/T
is
large
compared
with
1.
It is
easy
to prove
that
according
to
Planck's
theory
of
radiation, within the
range
of
validity
of
Wien's radiation
formula,
Ev/e
has
the value
e-Bv/T,
thus,
Ev
is
much
smaller than
e.
Therefore
only
a
few
resonators have
energies
different
from
zero.
In
my
opinion
the
above
considerations
do
not at
all
disprove
Planck's
theory
of radiation;
rather,
they
seem
to
me
to
show
that
with
his
theory
of
radiation
Mr.
Planck introduced into
physics
a new
hypothetical element: the
hypothesis of
light
quanta.
§2.
An
expected
quantitative relationship
between
photoelectric
diffusion and the Volta effect
It is well
known
that if metals
are
ordered
according
to
their
photo-
electric
sensitivity,
one
obtains
the Volta electric potential
series,
in
[18]
which
a
metal is
the
more
photosensitive
the
closer it is
to
the electro-
positive end
of the electric potential series.
[19]
To a
certain
degree,
this fact
can
be
understood
by
assuming
only
that
the forces
(which
are
not to be
examined here)
that
produce
the active double
layers
reside
on
the
metal-gas
interface rather than
on
the metal-metal
interface.