Here’s why:
The Circuit in 25.18b is effectively an open circuit. Modern volt meters can have input impedances in the billions of ohms (even the cheap ones are in the tens of millions of ohms). Because this circuit is effectively open, there is no current flow, so there is no voltage drop on the resister. Without a voltage drop, the source voltage is “read” by the meter.
There are many voltage sources that require a “load” to be accurately measured. Without a load, the source voltage may be several times higher than the specified rating.
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