How do you measure voltage of cell?

[jd12345]

How can you measure voltage - my friend confused me.
We all know we can do it by a voltmeter but a voltmeter is a galvanometer and we need to know the resistance of the galvanometer and current passing through it to find voltage
But to find out resistance we need to know current and voltage - and for current we need voltage and resistacne
turns out that we need the other two to determine the value--- then how do we measure the emf of a cell, resistance of a wire and current?

 

[kurtlesker]

just use a voltmeter (or a multimeter in voltmeter mode) directly on the two electrodes of your cell. This will give you open circuit voltage.

if you you have more than one resister, then you can find out how much voltage is dropped across either resistor by using a voltmeter across one the resistor in question

 

[NascentOxygen]

How can you measure voltage - my friend confused me.
We all know we can do it by a voltmeter but a voltmeter is a galvanometer and we need to know the resistance of the galvanometer and current passing through it to find voltage

It's true that the voltage you measure is not the exact open-circuit voltage of the cell, but for all practical purposes, it is as close as we care. Were there a need to be painstakingly exact, then you could make use of the specification sheet that comes with every voltmeter telling you how many kΩ/V resistance it presents at the terminals for its different voltage ranges. You can then make some calculations and slightly adjust your voltage measurement. https://www.physicsforums.com/images/icons/icon6.gif

 

[jd12345]

No no - this was not my question!

My question is a bit strange - perhaps hard to tell but please try to understand
So my question :-We make a voltmeter using a galvanometer and a high resistance and then to caliberate the galvanometer we need to know resistance of the galvanometer first! But then how do we find out its resistance - by doing the ohm's law experiement - by finding the ratio of V/I but for that we need to know the value of V - the thing we are trying to find!

 

[Dadface]

1.Resistance can be measured by use of a bridge circuit.
2.Voltmeters can be calibrated with comparison circuits and by using a power supply of known EMF.

For details google
1."Wheatstone bridge"
2 "Potentiometer circuits"and "standard cells"

These are time consuming,but depending on the actual circuitry/equipment used used can be very accurate methods.

 

[willem2]

No no - this was not my question!

My question is a bit strange - perhaps hard to tell but please try to understand
So my question :-We make a voltmeter using a galvanometer and a high resistance and then to caliberate the galvanometer we need to know resistance of the galvanometer first! But then how do we find out its resistance - by doing the ohm's law experiement - by finding the ratio of V/I but for that we need to know the value of V - the thing we are trying to find!

You calibrate a voltmeter by using a standard voltage reference, and adjusting the resistance until the meter produces the right value.

In the past standard batteries were used, and later zener diodes, but nowadays apparently a superconducting josephson junction.

http://en.wikipedia.org/wiki/Voltage_reference

 

[Bob S]

before 1960, we used a precision Wheatstone bridge to balance the unknown voltage against a Weston standard (wet) cell (about 1.02 volts) by "zeroing" the galvanometer.