Constantan
Manganin
60% Cu
&
40% Ni
84
Mg
12 Cu 4
Ni
a
a
0.0001
0.000011
Relatively minute admixtures of metals
or
metal oxides suffice
to
increase the specific resistance of
a
metal significantly.
The
following quantities
are
required
for
the
numerical
measurement
of the resistance
w,
l,
q,
t,
where
l
meas[ured] in
m
q
in
mm2
and t
in deg.
Celsius.
Thus, it remains
to
establish the unit of
w.
After the
most
different kinds of units had been used for
a
long
time, Siemens proposed the
following
as
the unit:
w
=
1
is the
resistance of
a
mercury
column of
1
mm2 cross
section and
1m
length
at 0°
Celsius.
Later
on one
undertook
to
fit this unit into the
c.g.s.
system
(Paris
Congress. This unit
was
established based
on
the caloric
measurements of Peltier's effect. It turned out that Siemens' unit
had the value
of 0.95.109 in the old system of
units.
Therefore,
instead of Siemens'
mercury
column,
an
analogous
column with
a
length
of 1.06
m
(later
on
corrected to
1.063)
was
established
as
the
practical
unit, which
was
named
1
Ohm
(ß).
Production of the Ohm.
Let
us
have
a
mercury
column in
a
glass
tube of
length
L
(in
cm)
and
cross
section
q
(in
mm).
We
prepare
a
metal wire with
a
resistance of
1
Ohm.
For that
purpose
one
chooses
a
metal whose specific resistance is
as
constant
as
possible,
say
one
of the
familiar
alloys.
The resistance of the
glass
tube is
We set
up
the familiar arrangement.
=
L
(in
cm)
q
in
mm
106.3
[Fig]
The resistance
x
shall become equal to 1.
W
=
l1
x
when
zero
current
in the bridge
x =
W
2
_ €i
q{106.3)'
Now
one
keeps varying
x
until the expression
on
the
right
hand
side
=
1
at
zero
current.
The resistance is then
x
=
1
We
have then
l1
L
q
(106.3)'
ß
If
one
has prepared
a
number of such wires of resistance 1, it
is
easy
also
to
prepare
resistances of other
mag[nitudes]
112