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I am just confused how can a voltmeter which has a very large resistance and hence small current or in ideal condition zero current still measure potential different because as far as I know voltmeter is modified galvanometer and a galvanometer shows deflection only if current passes through it.

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  • $\begingroup$ If a voltmeter had low resistance then most of the current would pass through it and it would stop functioning as a voltage reader. The shunt resistance is chosen such that it's high and the more current passes through it the more the galvanometer deflects giving us a measure of potential difference. Real voltmeters can never give 100% accurate reading and ideal voltmeter is only a concept. $\endgroup$ – Weezy Nov 2 '16 at 14:09
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Voltmeters come in many forms and as their implies they measure a difference in potential between two points.
One important characteristic of a voltmeter is that it does not alter the potential difference it is trying to measure and this usually means that its resistance is much higher than the resistance in the circuit where the potential difference originates.

For example if a current of $1$ mA is passing through a resistor of $1$ k$ \Omega$ then the potential difference across the resistor is $1$ volts.

Putting a voltmeter of resistance $1$ k$ \Omega$ across the resistor would mean that the current through the resistor would now be $0.5$ mA with the other half of the current passing through the voltmeter.
So the voltmeter reading would now be $0.5$ V.
However if the voltmeter had a resistance of $10$ M$ \Omega$ the volmeter would read $0.9999$ V because most of the current of $1$ mA would be flowing through the resistor and very little through the voltmeter.

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  • $\begingroup$ Then what is the basic principle that voltmeter works on? Do we measure potential difference by measuring resistance in voltameter and current in it and then calculate through formula V=IR? $\endgroup$ – AksaK Nov 2 '16 at 15:29
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    $\begingroup$ It all depends on the type of voltmeter. A moving coil meter works of the interaction of an electric current provided by the potential difference across the voltmeter passing through a coil and a magnetic field. This produces a torque on the coil and hence the coil is deflected. A digital voltmeter has inside it an analogue to voltage convertor and a successive approximation register. Although you may not realise it a gold leaf electroscope can be used as a voltmeter and then the deflection is produced by the repulsion of like charges on the gold leaf. $\endgroup$ – Farcher Nov 2 '16 at 15:38
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Most portable multimeters have an impedance of 10 MΩ on their voltage ranges (excluding "Low-Z" ranges). That is enough to allow a very small but measurable current to flow.

If the device under test also has a high impedance, this must be corrected for by the user - or a different measuring tool selected. See Burden Voltage for an example of the problems caused by a non-ideal real-world meter.

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