# Ohm's law and an ideal voltmeter

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.

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".

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

• 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. – CuriousOne May 28 '16 at 8:42

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.

• 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. – CuriousOne May 28 '16 at 8:20
• 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. – Zeeshan Ali May 28 '16 at 9:35
• 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. – CuriousOne May 28 '16 at 9:56
• Clearly you are not a teacher........ – Zeeshan Ali May 28 '16 at 10:01
• That's true, I am merely a person who actually knows how a voltmeter works. :-) – CuriousOne May 28 '16 at 10:04