# Stretched wire potentiometer, Conceptual help

1. Jul 1, 2012

### amk_dbz

I am having problem understanding the working of Stretched wire potentiometer.

Now the book says that the potential of a and A are same because the wire Aa has negligible resistance. That is were the problem comes for me. Suppose the batter connected to AB be of 10V (assume negative terminal to be of 0V) and the one connected to ab be 5V(assume negative terminal to be of 0V). Now the pt a is at 5V and A is at 10V, therefore current should flow from A to a, but this doesn't happen, Why?
Any help will be appreciated. Thank you.

(check the image of circuit diagram of potentiometer. Sorry for bad quality, I am not a good painter.)

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2. Jul 2, 2012

### Studiot

Are you sure you have copied the diagram completely and correctly?

A potentiometer measures voltage.
You cannot measure resistance with a potentiometer alone.

You can compare resistance with a known standard.

Or you can independently measure the current through the unknown resistance and use the potentiometer to measure the voltage across the unknown resistor and then calculate the resistance from Ohm's law.

Do you understand the basic operation of a potentiometer?

Last edited: Jul 2, 2012
3. Jul 2, 2012

### amk_dbz

I am a bit confused but what you said is correct. The potentiometer measures the potential difference and not the resistance (Sry for the mistake).

4. Jul 2, 2012

### Studiot

We can only help with more information.

5. Jul 2, 2012

### amk_dbz

I am a bit confused due to the problem I am facing as told in the first post. Other things are clear. So, could you help me with the problem? I haven't been able to find a satisfactory answer for it for a month or two, so any help will be much appreciated.

Thank you.

6. Jul 3, 2012

### Studiot

So is this your issue?

The wire that the potentiometer is made of is special wire with significant resistance per millimetre. The wire is not ordinary copper wire that is designed to have as little resistance per millimetre as possible.

When we connect two points (eg Aa) by ordinary copper wire we can say they are at the same voltage because the resistance of the connecting wire is negligible.

But when we connect say 10 volts across the potentiometer wire there is a regular voltage drop of 10 volts over 1000 millimetres or 1/100 volts per millimetre, or 1 volt every 100 millimetres. Since the voltage drop is regular (even) we can measure it by measuring distance along the wire.

So at a point at say 400mm from the zero end the voltage is 400 * 1/100 = 4 volts

If the voltage along the whole potentiometer wire is greater than the voltage you wish to measure then somewhere along the wire it must equal the that voltage.

So you can compare voltages by sliding a contact along the wire until the voltage at b and the slider (in your diagram) are equal.
Since they are equal no current flows through the galvanometer.

How are we doing? are we making progress?