# Electrical resistance of springs

1. Jul 3, 2010

### CuriousG44

I am attempting to measure linear spring displacement by measuring the change in electrical resistance of the spring before and after the displacement. Will this approach work? (assume the coils do not contact each other)

Last edited: Jul 3, 2010
2. Jul 3, 2010

Hello curious.If you were stretching a wire there would be a small change of resistance because the wire would get longer and thinner.Any change of resistance of a spring would be extremely small because the main displacement would be an uncoiling of the turns rather than a stretching of the wire.

Last edited: Jul 3, 2010
3. Jul 3, 2010

### Bob S

If two springs are placed in a galvanic cell with the appropriate battery electrolyte, there is a small electropotential difference created if two springs are used as electrodes, and one spring is compressed, due to the strain mechanical energy stored in the spring. Use fishline or other non-metallic fiber to hold one spring compressed. This might be an alternate parameter to measure..

Bob S

Last edited: Jul 3, 2010
4. Jul 3, 2010

Something else that springs to mind(sorry) is that stretching a spring will change its self inductance and reactance.

5. Jul 3, 2010

### dlgoff

That was my first thought when thinking about a way to measure CuriousG44s springs displacement. But that would require attaching wires that would possibly interfere (?). So I was thinking maybe some sort of linear optical encoder. There would be several ways to do it. The simplest would probably be hanging an encoder from the spring (note: there would be some initial stretching of the spring but since the displacement is linear it might not be a problem.) Then use an LED to reflect some light off the encoder and count the number of lines on the encoder. You could make the encoder resolution (lines/inch) to suite your needs. Here's how the concept works:

6. Jul 3, 2010

### Mike_In_Plano

I had a tech that once worked for me. He was hardcore experimentor, kept a lathe as well electronic equipment at home. Anyway, he came across an article regarding measuring spring displacement via capacitance. I think it was a NASA publication.

Anyway, built one with two the interleaved windings and a rubber coating and sure enough, it worked. It was a pretty usightly thing though and I don't know if it's consistency would be what you'd want over time.

Personally, I'd consider a piston and lined cylinder arrangement, or simply purchase a linear pot which is made for this.