# Ideal Voltmeter infintite resistance question

#### atomtm

Joined Aug 13, 2012
24
Hello everyone!
I was just reading chapter 8 - dc metering circuits and I was wondering what if you could actually get a voltmeter with infinite resistance , how would that suppose to work with no current flowing through it! How could a user get a measurement from it with no current producing a torque in the needle mechanism?

#### colinb

Joined Jun 15, 2011
351
The "ideal" voltmeter does not exist.

That's the difference between an ideal, theorical thing and any real implementation in practice: an actual realization of something theoretical will be non-ideal in one way or another. No exceptions.

#### davebee

Joined Oct 22, 2008
540
You can sort of do that with a nulling circuit. You set up a low impedance variable voltage of close to the same value as the voltage under test.

Connect the low impedance voltage output to the voltage under test through a sensitive ammeter, then adjust your low impedance voltage until you null the meter, so it shows zero current flow.

When the meter is nulled, then your low impedance source is the same voltage as the voltage under test, with no current being drawn from the voltage under test, and you can measure the low impedance voltage to get a value for the voltage under test.

That technique was common many years ago, but nowadays, I would suspect that there are op-amps with such sensitive inputs that the current they draw is so close to zero that they do a better job at this than a mechanical nulling ammeter would.

(edit - I should have read the article - nulling meters are discussed in that chapter)

#### crutschow

Joined Mar 14, 2008
27,400
Voltage can be also be measured by the electric field it produces, which can be measured without drawing current. For example, a MOSFET's operation is determined by the gate-source voltage which generates an electric field in the drain-source channel beneath the gate which controls the drain-source current. The gate looks like a capacitor, drawing only a minute DC leakage current, so it is a close approximation to the ideal.

#### MrChips

Joined Oct 2, 2009
23,896
Such an instrument is called an electrometer, i.e. it measures electric field while taking very little current.