not clear what you are doing, you want to detect vibrations ???
yes exactly, the vibrations, the speed of movement, the exact position, the pressure applied (other sensor for that) and maybe other parameters. It's to generate signals with a maximum of parameters.
you can't find the position of the finger with accuracy 0.25 mm by touching the wire unless the finger is less your finger is thinner than that.
It's more a precise delta position and a repeatability that interests me, of course define the finger position with this precision doesn't really make sense.
Don't know about speed.
A finger on a slider will also pick up (mains) hum, that has to be filtered out with a capacitor.
That filter could slow things down.
Did you consider conductive (carbon) paint.
You can (could) buy that in a spray can.
Or is this for some sort of string instrument.
Leo..
you need to describe in details what you are doing, right now is a guessing work.
As I said in the first message, I tried carbon on paper (pencil) but the result is not very linear, I think the result with the painting will be similar. I will, indeed, probably have to isolate the part in contact with the finger to avoid disturbances.This is also why the capacitive sensor could be a good idea too.
my problem is already solved (probably), I just need a resistance of more than 1K, long, flexible and linear. I will try with a ceramic resistance when I have one. just the idea of the capacitive sensor interest me too.
1k to reduce current draw...
If you can't find wire with high enough resistance,
then you could also drop supply voltage to the wire to e.g. 1volt.
And measure with the A/D connected to the same voltage (Aref to external).
Leo..
oh thanks, I didn't know this feature, but it lowers the resistance to only 200 ohms/m for a current of 20 mA, ideally I would like to have 4 or 5 of these wires on the same arduino. But I'll use that if I really can't do anything with ceramic resistances.
certicoms:
...but it lowers the resistance to only 200 ohms/m for a current of 20 mA
200 ohm with 1volt across is 5mA, not 20mA.
Leo..
but I count for 25 cm wires 
, they will probably do a little more but not much.
The track from a linear potentiometer ( commonly called a 'fader' when used for audio ) may do what you need. Various lengths and resistance values are available.
Conductive plastic ones are very good, but more expensive than carbon.
Use a length of thin strong wire eg nicr - (perhaps > 40swg) pressed against it as the pickup - it's resistivity doesn't matter much
Allan
I did not find the right size, but it must exist. I would also like to keep something flexible, but again, it will be a backup solution if the others do not work.
If this wire is 35cm long and 1000ohms in resistance, end to end, how does it detect the presence of a human finger? Is there a metal contact that travels the length of the wire that you move with your finger?
Hi,
Do you absolutely need to have conductivity through your finger?
Have you looked at FSR, Force Sense Resisitor?
google. Force Sensing Resistors
Hope it helps... Tom.. 
You mean like this: 500mm flexible membrane potentiometer at Sparkfun They have longer and shorter ones.
0.25mm accuracy is ridiculous. Unintentional changes in pressure will spread your fingertip out by more than that much.
MorganS:
0.25mm accuracy is ridiculous. Unintentional changes in pressure will spread your fingertip out by more than that much.
It's not ridiculous if you want to detect slight movement from side to side. Yes, maybe absolute finger position to 0.25mm is "ridiculous", but relative position is a different matter. If sliding the finger is to control, say, frequency, then such small resolution might be needed to get a smooth frequency change -- the ear can detect very minute frequency changes.
certicoms:
Hi,For a project, I have to find the position of a finger on a line. Like a touchscreen but on only one dimension.
Like a touch sensitive ribbon that they use for some instruments..... like the old korg prophecy?
Or it needs to be like a line like a guitar string?
Thanks for these new ideas, I don't believe that a force sensor can return a position. I will probably use one to have an extra parameter, but not for the position.
MorganS, it's very close to what I'm looking for, but because it is much more expensive I would first try with ceramic resistance. (Ribbon sensor is the same thing?)