Naked Science Forum
Non Life Sciences => Technology => Topic started by: JP on 28/07/2010 04:43:48
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I haven't done lab work in ages, but I seem to recall both names being used to describe things we used to measure voltage. (Don't hold me to that, though!) Wikipedia has a link describing the use of a potentiometer as a voltmeter. From reading the page briefly, it sounds like the potentiometer taps off (a variable amount) of current from a circuit to measure voltage, while a voltmeter isn't variable, and is usually designed to tap off as little current as possible. Am I right in thinking this?
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I haven't done lab work in ages, but I seem to recall both names being used to describe things we used to measure voltage. (Don't hold me to that, though!) Wikipedia has a link describing the use of a potentiometer as a voltmeter. From reading the page briefly, it sounds like the potentiometer taps off (a variable amount) of current from a circuit to measure voltage, while a voltmeter isn't variable, and is usually designed to tap off as little current as possible. Am I right in thinking this?
Erm, well, no!
A voltmeter measures voltage. The simplest form actually steals a little bit of current, and the current produces an electromagnetic deflection that is a function of the actual voltage. More sophisticated forms steal less, or almost no current at all.
A potentiometer (despite it's name) does not measure voltage, although it can produce a potential difference. It's simply a resistor with a sliding tap. The resistance between the outer terminals has a fixed value, and, depending on the position of the tap, a percentage of that resistance is presented between the tap and the outer terminals.
The Wiki reference in http://en.wikipedia.org/wiki/Voltmeter says:
"One may also measure voltage using a potentiometer in the null-balance method. The potentiometer's resistance is changed at the wiper until the null detector shows zero voltage between the two circuits."
Here's how the null-balance method works.
http://en.wikipedia.org/wiki/Potentiometer_(measuring_instrument)
In that situation, the potentiometer is not directly measuring anything. It is producing a potential (a voltage) and the galvanometer is used to indicate when no current is flowing, which means that the potential produced from the standard voltage divided by the potential divider (the potentiometer) is the same as the unknown voltage. Assuming the resistance of the potentiometer is accurately calibrated on a dial, the unknown voltage can then be derived from the position of the potentiometer.
So;
Can a potentiometer be used as part of a circuit that also includes a standard voltage and a galvanometer to measure voltage? If it's properly calibrated, yes it can, as can a lot of other things.
Can a potentiometer on its own measure voltage? No it can't.
Can a voltmeter measure voltage on its own? Yes it can.
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I haven't done lab work in ages, but I seem to recall both names being used to describe things we used to measure voltage. (Don't hold me to that, though!) Wikipedia has a link describing the use of a potentiometer as a voltmeter. From reading the page briefly, it sounds like the potentiometer taps off (a variable amount) of current from a circuit to measure voltage, while a voltmeter isn't variable, and is usually designed to tap off as little current as possible. Am I right in thinking this?
Is this the link you are refering to?
http://en.wikipedia.org/wiki/Voltmeter
"One may also measure voltage using a potentiometer in the null-balance method. The potentiometer's resistance is changed at the wiper until the null detector shows zero voltage between the two circuits."
Sounds to me it is an attempt to discribe a Wheatstone bridge. Null-balance method is my only clue.
http://www.scribd.com/doc/12936817/The-Wheat-Stone-Bridge-Lab-Report
It seems to need a word or two face lift.
Reference needs work too.
So much for Winkipedia... I need to watch out for and test the references given before believing.
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That makes a lot more sense now. Thanks.
The sentences that led me off track in Wikipedia were "A potentiometer is an instrument for measuring the potential (or voltage) in a circuit. The instrument taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer." I took it to mean that potentiometer was the measuring device which contained a galvanometer, which isn't the case.
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That reminds me of another important difference.
The other essential "component" in the circuit, is a human being! Someone has to adjust the potentiometer to find the null point of the galvanometer.
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Tommy,
Looks like our posts crossed in the night!
I found a good ref. on Wiki to the Null Balance Method - it's in my post above.
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The sentences that led me off track in Wikipedia were "A potentiometer is an instrument for measuring the potential (or voltage) in a circuit. The instrument taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer."
You know, it is possible that at one point in time a "potentiometer" could have described a mahogany box that included the whole bag of tricks (reference voltage battery etc.) Come to think of it, it's not impossible we had something like that in our physics lab in high-school, but if it even existed, I've no idea what we called it.
In modern electronic engineering parlance (last fifty years or so) a "potentiometer" only refers to a variable resistance of one sort or another. A more accurate handle might be "variable potential divider" but it's probably a bit late to try and change it.
Come to think of it, even the "whole bag of tricks" does not really measure voltage anyway. All it does is tells you the ratio between a reference voltage and an unknown voltage. It only allows you to measure an absolute voltage if you know the absolute voltage of the reference! How are you going to measure that? Probably with a voltmeter.
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That reminds me of another important difference.
The other essential "component" in the circuit, is a human being! Someone has to adjust the potentiometer to find the null point of the galvanometer.
Automatic, self-balancing potentiometers have been used. Good old fashioned chart recorders were based on them.
It's true that apotentiometer can only give you the ratio of to voltages, if oner of them is a standard cell then that's how you measure voltage.
This sort of thing.
http://en.wikipedia.org/wiki/Clark_cell
or
http://en.wikipedia.org/wiki/Weston_cell
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That reminds me of another important difference.
The other essential "component" in the circuit, is a human being! Someone has to adjust the potentiometer to find the null point of the galvanometer.
Automatic, self-balancing potentiometers have been used. Good old fashioned chart recorders were based on them.
It's true that apotentiometer can only give you the ratio of to voltages, if oner of them is a standard cell then that's how you measure voltage.
This sort of thing.
http://en.wikipedia.org/wiki/Clark_cell
or
http://en.wikipedia.org/wiki/Weston_cell
I'm sure our Physics teacher had some sort of voltage reference. It was something of a "sacred object" that was only used for particular calibration activities. Unfortunatly, my science notebooks are long gone!
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Tommy,
Looks like our posts crossed in the night!
I found a good ref. on Wiki to the Null Balance Method - it's in my post above.
Yep you hit the nail on the head.