OPERATING INSTRUCTIONS
MODEL
153
c.
To minimize the drift caused
by
t
h
e
r
ma
l
emf's, use the same
metal
or
metals having
lo� thermo-electric powers in the input circuit.
Gold, silver
and
low-thermal solder
have thermo-electric
g
owers within about
iO.2S
�v/oC of copper.
This
means
even
a tempera
ture difference of
10 C
between one of
these
metals and copper will generate a
thermal
emf
of
2.5
microvolts.
At
the other extreme, germanium has a
thermo-electric
p
ow
e
r
of
about
320
uv
/
oC, and silicon
will develop about
420
',J.v/oC against c
o
p
p
e
r
.
Standard
physica
handbooks
c
o
n
t
a
i
n
tables
of
thermo-electric
powers
of
m
at
e
r
i
a
l
s
.
Since
the
Model
153 input'
circuit is
of copper, the best
juntion
is
copper
to copper.
However,
copper
ox ide in
the
junction
or
differences
in processing
of two
pieces
of
copper can cause thermal emf's of
up
to
0. 2
microvolt per
DC.
The Model
1483
Kit contains all necessary equipment to
make
very
low-thermal joints.
See Section
7.
d.
Maintaining constant temperatures
also reduces thermal emf's.
For low
vol
tage measurements, keep all circuits
from
open windows, fans, air conditioning vents
and similar sources
which
vary
temperature.
Minimize thermal g
r
a
d
ien
t
s by
placing all
junctions physically close on
a
large heat
sink.
Thoroughly clean all copper
leads
with a
non-metallic
abrasive such
as
Scotch
Brite
before
making a
connection.
Crimp to
gether the ends of e
a
c
h copper wire;
bolt
the lugs
for
each connection
point together;
m
o
u
n
t
all
stacks of
lugs
on
a
thick metal
plate having high thermal conductivity.
Thermal
conductivity
between the junctions
and
the
heat sink
can be kept
at
a high
level by using mica washers or high conduc
t
i
v
it
y
ceramics for electrical
insulation.
e.
Several
other
techniques will reduce
the
effects of
thermal
e
m
f
'
s
.
Use
only
cadmium-tin
low-thermal
solder (Model
1503),
such as
supplied
in
t
h
e
Model 1483
Kit,
for soldered
connections.
Unlike
lRASS MUT
I."",,,,, sum
r--l
NYlON SPACERS
J
'
,i
COPPEl LUG
"
•
'
,-
.
I
I
__
:
.
IV
..
I
I
FIGURE
4.
Thermal
Sink Construction.
Connect leads or
lugs as close as possible.
Separate only with insulation of
high
heat
conductivity.
metals --- including regular solder
---
may
be used and low thermal emf's obtained if a
well
controlled oil
bath
or
a
good heat
s
i
n
k
is used.
Thermal voltages
may
be
calculated
from
the thermoelectric power of
the materials
in
the junction
and the
temperature difference
between
the junctions.
2-11.
SHIELDING.
a.
Generally,
the
Model
153
is
insensitive
to
ac
voltages
superimposed upon
a dc s
i
g
n
a
l
at
the
input terminals.
However,
ac
voltages which are
very
large
compared
with the de
signal
can cause
erroneous
readings.
Usually
it
is
suffi
c
i
e
n
t
to
connect
t
h
e
cases
of all
apparatus in
the
measurement circuit together
and
ground at one po
i
n
t
.
This provides
a
"tree" configuration, which
m
i
n
i
mi
ze ground loops.
Also floating
the i
n
str
um
en
t
(para
g
r
a
p
h
2-6)
will
minimize ground
loops.
The common pOint at which all shields
are
c
o
nn
ec
t
ed should
be
as near as
possible
to the
circuit low of
the
Model
153
at its
input.
b.
Improper shielding can cause the Model
153
to react in one or more
of
the
following
ways:
10
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