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In an ideal voltmeter there is infinite resistance so because of which no current is flowing through it and the voltage reading is not affected. The ohm's law stated that

V = I * R

In an ideal voltmeter I = 0 so V = 0 but the voltmeter does show some reading which defies the ohm's law. Please tell me how is this possible.

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It is possible to have a perfect voltmeter: You can use a potentiometer with a current meter but you also need something with a standard voltage (standard cell, whose emf is larger than what you are trying to measure). It is a null method, that works by varying the sliding contact on the potentiometer until a zero current is registered. Zero current means no pd across an internal resistance so no "lost volts". Have a look here for more details.

Apparently "This null balance measuring method is still important in electrical metrology and standards work and is also used in other areas of electronics".

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firstly V=IR is not ohm's law. it is the definition of resistance. ohms law states that V is proportional to I where R is a constant for ohmic conductors.

in a voltmeter a very tiny current flows through a large resistance and a much larger current flows through the device which has a smaller resistance such that V=iR=Ir

for ideal voltmeters the R tends to infinity whereas i tends to 0 but the product is still defined

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    $\begingroup$ The proper definition of resistance would be R=dV/dI (no matter how many textbooks teach this wrong). R=V/I is only useful when Ohm's law actually holds. Let us restate this, again, in a proper voltmeter there is no ohmic current flow. A proper voltmeter is either a potentiometric bridge trimmed to zero OR it's a device that behaves like a capacitor. $\endgroup$ – CuriousOne May 28 '16 at 8:42
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An ideal voltmeter is made from a galvanometer connected to a resistor ,whose resistance is considered to be infinitely big, in series. But in practical life you can never achieve this because ....well... you can never reach infinity.

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    $\begingroup$ That's not how good voltmeters work. They never worked this way, that's just the high school version of it that is being taught by teachers who don't know better. $\endgroup$ – CuriousOne May 28 '16 at 8:20
  • $\begingroup$ I completely agree with you but knowledge is supposed to be gained step by step....you cannot just thrust in high levels of facts into an unnurtured brain. $\endgroup$ – Zeeshan Ali May 28 '16 at 9:35
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    $\begingroup$ The OP gains nothing from you giving him false information about what a voltmeter is, which is not even "a high level fact". Do it right or don't do it. $\endgroup$ – CuriousOne May 28 '16 at 9:56
  • $\begingroup$ Clearly you are not a teacher........ $\endgroup$ – Zeeshan Ali May 28 '16 at 10:01
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    $\begingroup$ That's true, I am merely a person who actually knows how a voltmeter works. :-) $\endgroup$ – CuriousOne May 28 '16 at 10:04

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