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areas show where the ranges of two bridges overlap and
in the cross-hatched area all three bridges could be used.
Superimposed in this plot are heavy dashed lines
which show where an extra 0.1% error occurs on the C,
G, R, or L reading due to one of the frequency or D or
Q
dependent error terms (refer to Section
1.
5).
The numbers on the various lines refer to the ex-
planation below:
1.
end of the adjustment (full scale),
2.
first division of the adjustment (100% D or
Q
error),
3. 100% D or
Q
error (0.0005) at low frequency (no
C, G, R, or L error),
4.
the 0.001% (..!r-)2 error in Land C,
1
KC
S. the 0.5% D2 error in Land C,
6. the 0.1% D (1 f
kc
) error in Land C,
7.
the 0.002% (1
~c)
2
error in G and R,
8.
the 0.1% Q error in G and R,
9.
the 0.000001% (1
~c)4
error in G and R (this
error becomes large quickly above this line).
2.5.6
EXTENDING D AND Q RANGES AT LOW FRE-
QUENCIES.
Below 140 cps part of the DQ range is not
covered by any of the bridges of the Type 1608-A.
In
this range, an external adjustment can be used to extend
the D or Q range of the various bridges. For the C s ' C p ,
L s ' and L p bridges, this adjustment should be a decade
resistance box or a calibrated rheostat connected to the
OPERATING PROCEDURE
EXT DQ terminals of the bridge.
For the C p and R s
bridges, a decade capacitance box should be connected
from either EXT DQ terminal to chassis (the two termin-
als should be shunted together).
The readings on the external adjustments can be
converted
to
give D or Q by means of the following form-
ulas where R is in kD, f in kc, and C in
j.Lf:
C s bridge, LOW D
D
=
f(internal dial reading
+
0.942 REXT)
C p bridge, HIGH D (set internal dial to read 0.02)
D
=
1.091
f(REXT
+
0.536)
G p bridge, capacitive Q
Q
=
f(internal adjustment reading
+
41.9 C)
R s bridge, inductive
Q
Q
=
f(internal adjustment reading
+
41.9 C)
L s bridge, LOW Q
Q
=
f(internal dial reading
+
0.942 R)
L p bridge, HIGH Q (set internal dial to read (0)
Q
=
1.091
f REXT
2.5.7 CORRECTIONS FOR RESIDUAL AND LEAD IM-
PEDANCES.
At high frequencies, the errors resulting
from the residual bridge impedances and from the con-
necting lead impedances become more important, often
requiring corrections.
Corrections are given in Table
2-5.
These corrections give the first-order terms only,
and in the corrections, the measured value of the un-
known is assumed equal to the true value of the unknown,
and either value may be used to evaluate the error.
21
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