I have a conductive thread of about 2 feet (0.65 m). I would like to be able to sense the position of touch.
I was hoping that the resistance of the thread would change at constant rate along the thread. Is this true?
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If you put the thread across the terminals of a meter measuring resistance, no matter where you touch the thread there should be a small change in resistance but, unfortunately, that resistance change would be the same throughout the length of the thread - this on its own prevents you from achieving what I believe you want.
However, if you have two threads very close to each other but not touching you should be able to roughly transfer the voltage at the contact point to the 2nd thread. In effect, the 2nd thread becomes the wiper of the potentiometer but feed that 2nd thread into a high impedance amplifier or the DC voltage that your skin transfers will be severely diminished.
Alternatively use an AC signal across the 1st thread and therefore transfer the ac signal to the "wiper" thread - this will be more reliable I suspect.
You can use an ohm metre to see if the resistance per cm/inch is constant (does it increases linearly with distance), but to first order this should be true if it is an electrical wire.
There will always be some variation though, and this variation is one thing that will effect the resolution you have in determining the position of touch on the wire.
You could just characterize the thread by coming up with an R(x) equation model for the resistance along the wire, 'x' cm/inch from one end, using an ohm metre.
Btw, The Human Body Model for capacitance, as defined by the Electrostatic Discharge Association (ESDA) is a 100pF capacitor in series with a 1.5kΩ resistor, according to Wikipedia. So you might have an issue if you are trying to detect the human body load on a low resistance thread.