OK, got me on that one, although that is measuring the voltage across some resistance, which I suppose could be incredibly large. But certainly this is not a common setup.

This is a very common setup which we ask students to do in Physics labs, the Wheatstone Bridge.
The current drawn from the measured source is zero. Hence the resistance is infinite.

In this laboratory experiment, students learn how to use this potentiometer in order to measure voltage and resistance without using a DMM. This is the true origin of the word potentiometer = potential meter.

 

I think the really confusing thing about this subject is simply that the internal resistance of a given system is so often not mentioned.

From Wikipedia for example:

Because the atmospheric electric field is negatively directed in fair weather, the convention is to refer to the potential gradient, which has the opposite sign and is about 100 V/m at the surface, away from thunderstorms. There is a weak conduction current of atmospheric ions moving in the atmospheric electric field, about 2 picoAmperes per square metre, and the air is weakly conductive due to the presence of these atmospheric ions.

Using Ohms law of course we can clearly see that 100V at 2 picoamps actually implies an "internal resistance" of roughly 20 trillion ohms at surface level for the atmosphere. In other words you'd be lucky to be able to power a SINGLE LED using a mast mounted atop some structure nevermind an entire house!

 

When the meter is null then there is (ideally) no voltage across or current through the meter and its resistance looks infinite.

So, as long as you know the exact voltage, you can measure it without disturbing it!

Bob

 

So, as long as you know the exact voltage, you can measure it without disturbing it!

No, you don't need to know.
You just adjust the pot until the meter reads zero, and then measure the pot voltage (meter V in the post #17 diagram).

 

No, you don't need to know.
You just adjust the pot until the meter reads zero, and then measure the pot voltage (meter V in the post #17 diagram).

I know that. My point is that it does draw current during the nulling process.

Bob

 

A potentiometer as used in a Physics Wheatstone Bridge demonstration consists of a 1m resistance wire stretched along a 1m ruler.



A sensitive center-zero (-30-0-30 μA) moving coil ammeter is commonly used.


The experiment requires at least one known voltage reference. A voltage standard such as a mercury cell is often used. In our laboratories we have standard mercury cells that are over 50 years old. The measured voltages are within 0.01% of the voltage marked on the cell.




The experiment requires two measurements to be made.

1) The Vx unknown voltage source is applied. The slider contact (arrow) along the wire is adjusted until the ammeter shows no deflection. The position (Lx) of the slider is recorded.

2) The Vx source is replaced with the known voltage standard Vref. Again, the slider is adjusted until there is no deflection on the ammeter. The position of the slider Lref is recorded.

The unknown voltage Vx is calculated as Vx = Vref x Lx/Lref

No current is taken from the voltage sources Vref and Vx when the null position is attained.

The voltage of battery V1 is not required to be known. Resistance of R is adjusted depending on the range of voltages being measured.

 

My point is that it does draw current during the nulling process.

Which is generally of no consequence in doing the measurement.