Resistance amplification? 3

[timeline1968]

I have a project that I am working on that is used to show, very simply, the difference in resistance between different materials. I'm using wires of a relatively small diameter, however I'm seeing that the resistance is going to be so small that it will not be measureable using standard multimeters (~.04 ohm).

Is there some way that I can amplify the resistance for all these wires so that it will be measureable, and distnctly different by the same amount for every wire?

Any assistance would be greatly appreciated.

 

[melone]

Without highly sensitive equipment or custom circuitry, I doubt you will be able to see any real difference between those materials at room temp.

Heating the material is a very interesting idea. That would increase the resistance, but I'm not sure how much R would rise per degree. Still, it's a good idea!

The only other way to see the difference in Resistivity is to pass a large amount of current through each of your chunks of metal, and use a simple DMM / voltmeter to measure the voltage drop across.

Actually, this may be more desirable anyway. The following experiment will illustrate V=IR, and how each metal has a different resistivity.

1) Make large metal "bars" with each of your desired materials. 2 x 12 inches should work.

2) Attach a bar to a constant current source, and pass 10 amps through it. Obviously, make sure that the circuit is not exposed to the students, etc..

3) Measure the voltage drop across the bar at various points.

4) Calculate the R based on the known I and V.

 

[IRstuff]

So, how about using an instrumentation amp:

http://www.analog.com/en/prod/0,,759_782_AD524,00.html

With a gain of 1000, it'll crank the voltage drop up to something that a typical 3.5 digit DMM can read.
Or:

http://www.analog.com/en/prod/0,,759_782_AD620,00.html

Which requires external resistor to set gain up to 10000.

They both list for <$10 in 1000 quantities, but the latter lists at $3.14. There are cheaper ones that AD sells that might also work.


TTFN

FAQ731-376

 

[unclebob]

Think I got it! Why not use a micro-ohmmeter! Service guys call it a ducter. With a sufficient lenght of wire, 'bout
5-6 feet long, it may be sensitive enough for the job. Set at 10 Amps, it can read 10e-6 Omh.

We use it to measure resistance of MV busbar junction
(among other things), where conducting surfaces are large to provide very low resistance...

It should be easy to get, just have to rent one.

 

[OperaHouse]

Instrumentation amp! Any crummy op amp would do it. Just need to adjust the zero pot every minute. Circuit board would cost more than the parts. Again, not practical for a run of 100.

 

[IRstuff]

An IA is not subject to user twiddling or operator error, since it's completely self-contained.

TTFN

FAQ731-376

 

[timeline1968]

Forgive my lack of knowledge regarding IA's, but it appears as though they need a voltage supply to work, in the first case 18V? Is this correct?

 

[IRstuff]

It's possible to run the current source from a D-cell, and the gain stage from a couple of 9V batteries. The minimum supply voltage on the IA is +/-6V

TTFN

FAQ731-376