294
THE
RELATIVITY
PRINCIPLE
force effects of
electromagnetic
fields
on
electric
masses,
we can
now
restrict ourselves
to determining
the transformation
equations of
the
latter.1
Let
the
quantity of
electricity
e
be
at
rest
with
respect to
S'. In
accordance
with
equations (12),
the force
acting
upon
it is then
given
by
the
equations
*
=
el
x
K
X
a
v
¥
- -
N
K
=
e
Z
+
^
M
c
Kz
=
iZ
From
these
equations and equations
(7a)
it follows that
pv
Kz
-h
=
ß-K
-
ß-KZ
y
(21)
These equations allow the
calculation
of
forces if
they
are
known
with
respect
to
a
co-moving
reference
system.
We now
consider
a
pressure
force
acting
on a
surface
element
s' that
is
at
rest
relative
to
S':
=
p1s'•cos
t
'
=
K\
-
p}
'S]
«cos
/»'
=
y
p
z
=
p]
-s1*
cos
n]
=
p]°sx
T)] '
S
f
F
y
p'-S'z
1This
circumstance
also
justifies
the
procedure used in the
preceding
investi-
gations,
in
which
we
introduced
only
interactions
of
a
purely electromagnetic
nature between
the
system
considered
and
its
surroundings. The
results
are
valid in
general.