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Why this kind of variable resistor is called a "meter", when it is not a measuring instrument?

I have seen another instrument of same name. In our school physics lab, there was a big rectangular, board-like, measuring instrument called a potentiometer, used to measure resistance.

Did the name of the 'electronic potentiometer' came from that 'measuring potentiometer'? If so, then are there other relations (not only the relation in name). What are the relations?

Diagram of a school-physics lab potentiometer, used in measurement purpose Fig: The diagram given, is the diagram of that measuring instrument from my old school textbook. AB is a long, uniform, cylindrical conductor rod (attached to a Wooden scale M) , on which there is a "Jockey" (wiping contact point). On A and B, voltage is applied using the battery E. (The commutator C used just to rapidly reverse the polarity). Galvanometer G is used to detect a null point. The scale M is used to measure the ratio of AJ and JB.

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    \$\begingroup\$ I didn't downvote it myself, but my guess is the downvote is because you could have learned all the answers by simply going to Wikipedia and typing "potentiometer" in the search box. \$\endgroup\$ – Dampmaskin May 15 '16 at 15:07
  • \$\begingroup\$ this is not a 'high value question' I expect it will be closed unless you make the question more useful. \$\endgroup\$ – Sean Houlihane May 15 '16 at 15:29
  • \$\begingroup\$ thanks. i searched wikipedia, but did not found (may be missed) why this variable resistor is being called a meter. Any guideline to improve This question? \$\endgroup\$ – Always Confused May 15 '16 at 18:40
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    \$\begingroup\$ @rdtsc Of course it is a meter. That's the entire point. A common application of potentiometers is controlling audio volume by measuring absolute degree of rotation of a shaft ratiometrically, relative to a minimum and maximum degree of rotation. They're called volume knobs. They work by measuring the rotation of the knob. Or metering it. Even if the reasons for the name 'potentiometer' are entirely historical, saying it is not a meter is incorrect. \$\endgroup\$ – metacollin May 17 '16 at 1:20
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    \$\begingroup\$ How is this "Primarily Opinion Based"? The etymology of a part of common electrical engineering is firmly historical and factual, not an opinion. \$\endgroup\$ – Passerby May 19 '16 at 2:15
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First, to address the name issue- in ancient times there was an instrument called a potentiometer.

I (being somewhat 'experienced' myself) have actually used them for serious work such as calibration of hundreds of control instruments, though they probably mostly can be found in museums now. It was used to measure (meter) voltage (potential) using a voltage divider as described, a reference and a galvanometer. I suppose our ones were relatively small so we called them 'portable potentiometers' or 'portable pots'. They were made by the instrument leaders of the day- Leeds and Northrup, Biddle, Kent and some by less known ones such as West.

enter image description here

In using the potentiometer one would first balance the potentiometer to a Weston reference cell (a sort of very high precision primary battery with various toxic substances- cadmium and mercury inside a glass structure). The cell was stable over time, temperature and was very long life if you didn't draw current from it. The galvo was zero'd at zero current and only briefly (push-button). Once that was done you would switch over to the input and zero the galvo against the voltage produced by the divider and read the potential off the calibrated scale. Power for the divider was supplied by primary batteries (such as relatively stable mercury D cells). Good ones had fancy galvos with mirror optical levers to give exquisite sub-microvolt sensitivity.

In more recent years it was most used for very low voltage low impedance sources, so it was used into the 1970s and 80s for thermocouple work. They have been replaced by cheaper devices- but the stability and ruggedness of those old wire-wound resistor assemblies was very, very good, and virtually no thermal EMFs due to careful choice of materials and careful assembly methods.

Now we call the variable voltage divider part a 'potentiometer', and the instrument has been mostly forgotten except by us more 'mature' folks. Perhaps similar to the way we 'dial' a phone using a touch screen.


With regard to your other questions- a pot divides the voltage between the two end terminals depending on the position of the wiper, so it can be used with any two voltages (within voltage and power dissipation limits). Usually we try to avoid drawing much current from the wiper, for various reasons. You can also connect the pot with 2 terminals only as a rheostat ('rheo' for flow, as in current, 'stat' for constant), which is essentially a variable resistor. In that case, all the current goes through the wiper. The unused end of the pot element is best connected to the wiper, which results in a slight improvement in performance.

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  • \$\begingroup\$ Thanks very very much about informing me about the basics and origin of potentiometer. By the way, do-you mean it was called 'potentio'- meter because it was used to fix the 'potential difference' obtained from that battery (Weston ref. cell)? please write a bit more clearly? so that i can make a the proper sense. \$\endgroup\$ – Always Confused May 15 '16 at 19:16
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    \$\begingroup\$ The potential in question is the unknown one being measured. The pot (powered by mercury cells) is calibrated with the Weston cell, then a known potential is created and that adjusted and compared with the unknown potential until the galvo does not move. Then the known potential is read off the calibrated scale. It's more complex than that, there are dividers and such like for different ranges, but that gives an idea. \$\endgroup\$ – Spehro Pefhany May 15 '16 at 19:55
  • \$\begingroup\$ Feeling Lucky with the phrase "Now we call the variable voltage divider part a 'potentiometer'," . Thank you so so much. \$\endgroup\$ – Always Confused May 16 '16 at 9:42
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    \$\begingroup\$ Good answer. I want only add some link that could be very useful for better comprension about how was used for measure potentials: * this book, Instruments of Science: An Historical Encyclopedia. See, in particular Clark method; * Clark potentiometric scheme in italian but schematics and formula are comprensible (and useful). \$\endgroup\$ – Antonio May 17 '16 at 7:51
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    \$\begingroup\$ The good old days \$\endgroup\$ – user132236 Dec 17 '16 at 23:15
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1) There are three legs because a potentiometer is a voltage divider. If you use only the middle and one outer leg, you are using it as a rheostat, which is also a valid way to use it.

2) How to connect, depends on the circuit.

3) You can use it as a voltage divider between any two voltage potentials, see 1).

4) See Wikipedia: potentiometer

5) (The rest of the question): See 4).

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  • Why there are 3 legs in a potentiometer?

    That's how a potentiomenter works. It's a three-terminal device. Think of it as a resistor with a adjustable tap. The resistor has two ends, and the tap is a third terminal.

  • what would be the problem, if a variable resistor with only 2 Legs used?

    None, although then it's not a potentiometer anymore, just a variable resistor. Potentiometers are sometimes used this way by not connecting one of the end terminals.

  • How to properly connect the 3 Legs of a potentiometer in a circuit?

    That is circuit dependent. This Q&A format is not suitable to a long introduction to how and why you'd use a pot in various circuits. You need to learn some circuit theory before any reasonable answer here is possible.

  • Is the third leg always conserved for ground (negative) only?

    No. There are many ways to use a pot.

  • Why this-kind of variable resistor is called a "Meter"

    It's not, so this question makes no sense.

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  • \$\begingroup\$ Thanks. I'll agree that, to properly understand the connections of pot, I need to learn the circuit theories at first. However, the word potentiometer ends with 'meter', so i asked that. \$\endgroup\$ – Always Confused May 15 '16 at 18:53
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is there any relation of that-type of potentiometer with electronic, 3-lead variable-resistor potentiometer?

What do you think the device labelled "M" in your diagram (with terminals "A", "B", and "J") is?

Edit

Okay, two downvotes, so I'll spell it out for people who don't like to think for themselves.

The device apparently labelled "M" (okay, the caption makes clear that "M" is only supposed to refer to the visual scale stuck to it, but that's hardly clear from the image itself) is a resistor with a third terminal connected to a wiper that can be moved to make contact at different positions along the resistor.

It is exactly the thing that is called a potentiometer today.

For a similar situation where a word is used today to mean only part of the device it used to refer to, see "battery".

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  • \$\begingroup\$ I've added caption to the diagram. \$\endgroup\$ – Always Confused May 16 '16 at 9:45

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