Section 4
4-2
4.1.1. The automatic direct-current-comparator resistance bridge.
4.1.1.1. The manual bridge - background.
The direct-current-comparator bridge is a resistance bridge particularly suited for the
comparison of "low value" resistors and the scaling of low resistances under
conditions in which each resistor functions at its own power level. In this bridge
network, the ratio of the resistances is determined from a knowledge of the ratio of
the currents flowing in the bridge resistors.
The direct-current-comparator, shown schematically in Figure 4-1, provides a means
for connecting together two isolated current sources with two resistors and the two
ratio windings of the current comparator. The current comparator is a current ratio
indicator based on the detection of zero flux in the magnetic core of the device. For
direct current applications a double core magnetic modulator is used which consists
of a double toroidal core magnetic modulator and a magnetic shield. The two ratio
windings link the modulator and the magnetic shield which carries the currents to be
compared. When the modulator output is zero the current ratio is equal to the turns
ratio to a high degree of accuracy.
At ampere-turn balance,
(Nx) x (Ix) = (Ns) x (Is)
For the manually operated direct reading bridge (model 9975), a second bridge
balance condition is necessary to make the resistance ratio result direct reading. This
second balance is achieved by manually adjusting the number of turns Nx until the
output from the bridge null detector is zero.
At the null voltage condition
Vd=0
and (Ix) x (Rx) = (Is) x (Rs)
When a flux and voltage balance is achieved it is clear that:
Rx = (Rs) x ( Nx / Ns )
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