DOC.
47
297
E
=
&
1
-
£
ff
=
1
-
S
1E°
=
q
C
E
[78]
where
E0
denotes the
energy
of
the radiation with
respect to
a
co-moving
reference
system.
However,
if the walls of the
hollow
body are
completely
flexible
and
extensible,
so
that
the radiation
pressure
exerted
from
within
must be
balanced
by
external forces exerted
by
bodies
not
belonging
to
the
system
considered,
one
has
to apply equations
(16c)
and
(18c)
and insert into
them
the well-known
value of
the
radiation
pressure
1
E
Po =
3
'
[79]
so
that
one
obtains
E
=
4
+
3
%
Ii
-
£cl
4
r
h
-
£
c2
c2
Next
we
consider the
case
of
an
electrically
charged
massless
body.
If
external forces
do
not
act
upon
the
body,
we
can once
again apply
formulas
(16a)
and
(18a).
Denoting
the electrical
energy
relative
to
a co-moving
reference
system
by E0,
we
get