and
2
3 4
1
1
1
2
3 4
by connecting them in series and in parallel.
Siemens
devised
a
convenient
assembly
of resistance units
according
to
the
following
scheme.
[Fig.]
Using
metal
stoppers
it is
possible
to
produce
resistances of
magnitude
1
-
10.
To produce
resistance wires,
one
again
uses
alloys.
The measured
value has to be corrected for temperature.
If the
res.
is calibrated
at 0°, then
W
=
W'(l
+
at
)
,
where
W1
is the resistance read
off,
and
W
the actual resistance. To avoid induction effects and
errors
of
measurement associated with them
as
much
as
possible,
the wire is bent
in the indicated
manner
before winding and is taken double.
[Fig.]
r,ß
e
V =
(a)
•
-
In practice,
l
is us[ually]
measured in meters and
q
in mm2.
The
sp[ecific] resistance depends
strongly
on
the chemical
purity
of the substance.
E.g.
Cu
+
Cu
+
Cu
+
1
TÜ00
1
1000
1
10000
Zn....10..5* increase of
w
Pb.
. .
11*
60-70*
II II It
It II
1
An
example
of the difference in resistances between
pure
metals
&
alloys.
1
part
platinum
plus
2
parts silver produce
an
alloy
of
w
=
0.24
II
w
,
..
=
0.059
platinum
=
0.0187
Ag
The connection between electrical
and
thermal
conductivities
of
pure
metals.
If
one
measures
thermal conductivity
using
the units
gr
cm
minutes
1°
113