illustrated in Figure 5-12. Here the reference voltage is represented as a

battery. A calibrated voltage divider, R1, provides an adjustable output.

The unknown input voltage is connected to input terminals A and B. When the

voltage divider is adjusted so that the two voltages are equal, there will

be no current flow in the meter.

This condition is termed a null.

The

magnitude of the unknown voltage is equal to the product of the reference

voltage and voltage-divider ratio. In a practical differential voltmeter,

the voltage-divider controls are calibrated in terms of the input voltage.

Typical accuracies of this type of dc voltage measurement are of the order

of 0.005%.

Figure 5-12.

Simplified differential voltmeter

2.

The principal elements of the Model 887A Voltmeter are depicted in the

Functional Block Diagram of Figure 5-13.

In this diagram, the Reference

Supply battery of Figure 5-12 has been replaced by the Reference Power

Supply; the voltage divider has been replaced by the Kelvin-Varley Divider;

and the Meter has been replaced by the Null Detector.

Additionally, the

input circuit has been modified to include an AC to DC Converter and a DC

Attenuator.

Figure 5-13.

Functional block diagram, Model 887A/AB

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