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If I have a circuit like this and measure the voltage drop across the resistor I get a value, regardless of what resistor I use:

Resistor Circuit

However, if I stick a piece of wood in there instead of the resistor I get nothing. My intuition says there must be some resistance value between wood and conductor where the voltmeter switches from reading the voltage source to reading zero. Is that how this works? Is there some high resistance value where the voltmeter reads a fraction of the source voltage? Is my understanding of electronics terribly wrong?

EDIT: I suspect the answer has to do with current and the voltmeter not being an ideal, infinite resistance machine, but I can't get from there to the answer.

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    \$\begingroup\$ as long as current exists, so does a voltage drop. \$\endgroup\$
    – hassan789
    Mar 4 '14 at 2:11
  • \$\begingroup\$ Usually we measure a voltage drop across a component. \$\endgroup\$ Mar 4 '14 at 2:21
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    \$\begingroup\$ In your schematic, you've got an apparent voltmeter connected like an ammeter. Since an ideal voltmeter is an open circuit, there is no current through the resistor (i.e., you don't actually have a circuit) and thus, there is no voltage across it. Said voltmeter should read the source voltage for any finite resistance. If you want to measure the voltage across the resistor, replace the voltmeter with a wire and place the voltmeter in parallel with the resistor. \$\endgroup\$ Mar 4 '14 at 2:31
  • \$\begingroup\$ Sorry if I was unclear, that was exactly my question - "Said voltmeter should read the source voltage for any finite resistance". What is finite? Isn't an insulator still a large, but ultimately finite resistance? \$\endgroup\$
    – Nate
    Mar 4 '14 at 2:38
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If your voltmeter is digital, it has a least significant digit that will read zero until you overcome that threshold. It also has a finite input resistance, for example say 10 megOhms.

So it's a simple voltage divider problem. The meter resistance/(wood or whatever resistance + the meter resistance) has to be greater than one least significant digit to cause the meter to read anything.

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I suspect the answer has to do with current and the voltmeter not being an ideal, infinite resistance machine, but I can't get from there to the answer.

Basically. It's called precision. Even the most precise voltmeters have a limit to how small a voltage they can read. The better designed, the smaller the voltage. And as you mention, they are not ideal. The voltmeter's accuracy can depend on the voltmeter's power source voltage, or temperature. The circuit adds resistance, as does the voltmeter wires.

The exact value of where any given voltmeter stops reading a voltage depends on the specific voltmeter design, and even individual ones in a batch.

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    \$\begingroup\$ The parameter of the voltmeter that is important here is not called precision but input impedance (resistance), as John D explains. \$\endgroup\$ Mar 4 '14 at 17:00

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