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For
low-resistance measurements, short,
heavy
leads should be used as connections to the unknown
component.
Measure the zero resistance of the lead!';
and terminals by connecting the free ends together, and
subtract this amount from the bridge reading with the un-
known in place.
For best connection to the bridge,
screw the binding post hard enough to notch the wire
inserted in the hole.
2.3.3
INTERNAL VOLTAGE APPLI ED TO THE UN-
KNOWN. There are three internal dc supplies, each hav-
ing a limiting resistor to limit the available power
to
1/2 watt or less to avoid damage
to
the bridge compon-
en~s
or to the unknown.
They are all controlled by the
GEN LEV panel control. The lowest voltage supply, ap-
proximating 3.5 volts open circuit, is
appli~d
"horizon-
tally" to the bridge (see Figure 1-3) and the 35-volt and
350-volt supplies are applied "vertically".
The FULL
SCALE RANGE switch selects the optimum supply for
each range as given in Table 2-1.
Because of the limiting resistor, the maximum
voltage applied to the unknown is usually of much less
than the open-circuit value.
Figure 2-1 shows the ac-
tual voltage applied to any unknown resistor when meas-
ured on the R bridge (with a Il5-volt line voltage).
EIA specifications for testing different types of
resistors
are summarized in Tables 2-2 and 2-3. Figure
2-1 shows thar these standard voltages can be supplied
from the internal power supplies over most of the resis-
tance range.
For low-resistance measurements the GEN
LEV control can be set for the desired test voltage by
use of a high-impedance dc voltmeter connected directly
LO
the UNKNOWN terminals.
For high-i:esistance meas-
urements, where the voltage is applied vertically, the
ratio between the voltage across the unknown and that
across the whoT~ bridge is fixed over each range at null
and therefore the voltmeter can be placed across the
bridge input (LOW UNKNOWN terminals to chassis) and
the GEN LEV control set to give the "bridge voltage"
given in Tables 2-2 and 2-3.
2.3.2 ACCURACY.
The accuracy of dc resistance and
conductance measurements is
±0.1%, ±0.005% of full
scale (which is ±1/2 of the last digit) on all but the
lowest R and highest G ranges as long as there is suf-
ficient sensitivity. On the lowest R and highest Grange
the accuracy is limited by the sensitivity to
±
1/2%
±1
mD.
2.3.4
EXTERNAL DC DETECTOR.
The internal dc
supplies and the internal detector permit measurements
from 1 ohm to 1 megohm
to
0.1 % when the GEN LEV and
DET SENS controls are at maximum.
If
accurate meas-
urements beyond this range are desired or if it is neces-
sary to make measurements at lower voltages, an exter-
Figure 2.1. Dc voltage applied to unknown
resistor (115-v line).
10
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