102
DOCS.
143,
144
MARCH
1909
your
treatise
on
ponderomotive forces,
as
well
as
the
book
on
the
electron
theory,
all
by
yourself.
I
have
been rather
unwell
lately,
and
am
still
suffering
from
a sore
throat.
I
have not
received Abraham's
paper,
but
I
am
not
in
a
hurry
to
read
it;[3]
I still
consider
our ponderomotive
forces
correct from
the
point
of
view of
the electron
theory.
My
Maschinchen
is
now
back from
the
mechanic[4] at
last,
and
I
am
busy doing
experiments;
I
now
have
ready
at
hand
everything
that
I
need
for
investigating
the limits
of
applicability
of this
method.[5] I
will
carry
out
an
investigation
of the
contact and
coherence
question
with
Guillaume.
Greetings
from
your
A.
Einstein
All
the others
also
send
their
greetings.
144.
To
Albert
Gockel
Bern,
Thursday [25?
March
1909][1]
Highly
esteemed Professor
Gockel:
Pursuant
to
your
kind
invitation,
I
shall arrive in
Freiburg
Sunday
at 9:05 provided
I
do
not
hear
from
you
in the meantime.[2] I
fully agree
that
we
should
do
the
experiments
with the
mercury
contacts at
hand. If
you can
spare
a
little time for
preparing
things,
then
I
would ask
you
for
the
following:
1)
To
set
the ebonite
disk in such
a way
that
all
of the
little wires
can dip
into
the
beads of
mercury.
2)
To
have
gasoline on
hand
for
cleaning
the
ebonite
parts
3)
To maintain
an
electrometer
in
working
order, with which
we can measure
0.001
volts.
The latter
is
especially
important,
because
we can
then
hope
to
be able
to
determine
the
sensitivity
margin
of
error (because
of the
contacts)
achievable
by
the
existing
means.[3]
With
best
regards,
yours,
A. Einstein
I
already
know from
my
experiments
that
the
thing can
be
done
with
the
mercury
contacts.
It
is
only
somewhat
time-consuming
to
put
the
apparatus
in
working
order,
and
the
mercury spurts
out
as soon as one
turns
a
bit
too
fast.
These
are
the
main
reasons
why
I
would
prefer
solid
contacts.
There
is,
in addition,
a
theoretical
reason
to
be
considered,
namely,
the circumstance that the continuous renewal
of
the
surfaces
contiguous
with the
air
will
produce
relatively large charges.