Thread: Percent of a Composite from Resistivity values

1. I am working on a resistor lab. I found the resistivity of 5 resistors all made of the same material, a silver-nickel composite. I would like to find the percent silver/nickel of the wire.

Average Resistivity: 4.56e-7
Resistivity of Silver: 1.59e-8
Resistivity of Nickel: 6.99e-8

Here is my equation:
Ω_silver (percent)+ Ω_nickel (100-percent)= Ω_composite

With everything substituted:
[1.59×〖10〗^(-8) ][%]+ [6.99×〖10〗^(-8) ][100-%]= 4.56×〖10〗^(-7)

But this gives me % = 121.

Now I realize that these resistivity values are at 20c and my lab was warmer and that the wire most likely has some other materials in it, but shouldn't I be able to get a reasonable answer?

What am I doing incorrectly?

P.S. Sorry for the simplicity of this question in comparison to all the other questions here. This was the first place that showed up on Google. Thanks for your help.

2.

3. I'd have thought the resistivity of the composite would be somewhere between the two metals it is made of. Your data shows it is higher than either one, so naturally you won't get a reasonable answer.

4. Originally Posted by Harold14370
I'd have thought the resistivity of the composite would be somewhere between the two metals it is made of. Your data shows it is higher than either one, so naturally you won't get a reasonable answer.
Right, that's the problem I was having; it doesn't make any sense. I've double check my data and calculations, even checking with others, so my average is correct with the data I had. What do you think is the most reasonable explanation?

1)Other metals in the wire of the resistor
2)Temperature Variation
3)Human reading error (Amperes/Volts)

It's probably a combination of all three and more. It just seems like my answer is too off. I tried the experiment with a copper resistor and got 1.71e-8 which is really close to 1.72e-8 (resistivity of copper). So I can't imagine the temperature has much to do with it (it's only a few degrees) and I'm pretty sure that the data was collected properly. So that really only leaves impurities in the wire. Does this seem accurate? Again it just seems like my answer is too far off? Ideas?

Thanks for your help.

5. Resistivity and resistance is a molecular structure kind of thing.

Perhaps this alloy formed from the two metals has a significantly different molecular arrangement that alters the resistivity to a higher value than either pure metal

6. Originally Posted by organic god
Resistivity and resistance is a molecular structure kind of thing.

Perhaps this alloy formed from the two metals has a significantly different molecular arrangement that alters the resistivity to a higher value than either pure metal
Sounds plausible. I don't know anything about this; can anyone confirm this theory?

Edit: I see that Harold14370 is the Mod of the Electrical/Electronics category. Perhaps, I ought to ask someone there about the material in resistors. Thoughts?

7. Nichrome wire is like that (80 percent nickle, 20 percent chrome) 108.0 ÷ 10^6, ohms * cubic centimeters.

Yet nickle 99.4 pure is 9.5 ÷ 10^6, ohms * cubic centimeters.
And Chromium is 13 ÷ 10^6, ohms * cubic centimeters.

I show silver as 1.59 ÷ 10^6, ohms * cubic centimeters.

I had some copper wire, with a nickle and maybe chromium electroplating, it was heater supply wire, that showed no ohms on a 500 foot roll, with one ohm meter I used. That is pretty odd I thought.

Sincerely,

William McCormick

8. How consistent was the resistance of each of the 5 wires you tested?

One possibility is that there is an oxide film on the resistors so that you are not making good contact with your ohmmeter probes.

Another possibility is the thermocouple effect. There is a voltage at the junction of dissimilar metals which is a function of temperature. The copper leads of your test equipment will form a thermocouple with the silver/nickel material at each connection. This shouldn't matter if the two junctions are at the same temperature, because the voltages will cancel out. But if the resitor wire is long, there could be a temperature difference from one end to the other. That's probably not the problem - just throwing that out there for you to think about.

9. Originally Posted by William McCormick
I show silver as 1.59 ÷ 10^6, ohms * cubic centimeters.
Just FYI, I'm in ohms*meters.

If Silver and nickel act in the same manner, then I suppose my math doesn't work. Can anyone specifically state that Silver/Nickel acts this way. Does anyone have a descent Table of Resistivity with composites?
Originally Posted by Harold14370
How consistent was the resistance of each of the 5 wires you tested?
I'm assuming you mean resistivity; the resistance was all over the place because of different lengths.
4.50×〖10〗^(-7)
4.59×〖10〗^(-7)
4.51×〖10〗^(-7)
4.65×〖10〗^(-7)
4.54×〖10〗^(-7)

Originally Posted by Harold14370
Another possibility is the thermocouple effect. There is a voltage at the junction of dissimilar metals which is a function of temperature. The copper leads of your test equipment will form a thermocouple with the silver/nickel material at each connection. This shouldn't matter if the two junctions are at the same temperature, because the voltages will cancel out. But if the resitor wire is long, there could be a temperature difference from one end to the other. That's probably not the problem - just throwing that out there for you to think about.
Well, the lengths of the Silver/Nickel aren't too long.
.80 m
1.20 m
1.60 m
2.00 m
2.00 m

The Copper was 20.00 m.

Obviously, I was using some other wire in the circuit to connect all the equipment together. I don't know what it was made of, but it was well insulated and rather thick. The only thing is that this can't be the problem because the copper was right. Does this sound valid? Just thought I would mention it.

Originally Posted by Harold14370
One possibility is that there is an oxide film on the resistors so that you are not making good contact with your ohmmeter probes.
That would be possible as I'm not quite sure how long these resistor have been around.

Thank you all for your input.

10. There is no alloy of nickel and silver listed on the Wikipedia list of alloys.
http://en.wikipedia.org/wiki/List_of_alloys
There is something called nickel silver which is actually nickel and copper and does not contain silver. Could that be what you are dealing with?

The copper wire would not have a thermocouple effect if your test leads are copper and so are the resistors. There would be no volatage at a copper-copper junction. I doubt that's it anyway since the resistivity you measured is pretty consistant and the thermocouple effect would be somewhat random depending on the temperature differentials.

11. Originally Posted by Harold14370
There is no alloy of nickel and silver listed on the Wikipedia list of alloys.
http://en.wikipedia.org/wiki/List_of_alloys
There is something called nickel silver which is actually nickel and copper and does not contain silver. Could that be what you are dealing with?

The copper wire would not have a thermocouple effect if your test leads are copper and so are the resistors. There would be no volatage at a copper-copper junction. I doubt that's it anyway since the resistivity you measured is pretty consistant and the thermocouple effect would be somewhat random depending on the temperature differentials.
Even if it is nickel silver (which I don't think it is, but I don't have the resistors with me), I still have the problem of the "out of bounds" average. Would the molecular structure effect that?

I found a table here, but I don't know how it applies.

12. Here is a table of AgNi alloys with resistivity values.
http://www.alibaba.com/product-gs/51630546/AgNi.html
It looks like the resistivity is supposed to be between that of silver and nickel, as one would expect. So this doesn't help explain your results.

13. Originally Posted by Harold14370
Here is a table of AgNi alloys with resistivity values.
http://www.alibaba.com/product-gs/51630546/AgNi.html
It looks like the resistivity is supposed to be between that of silver and nickel, as one would expect. So this doesn't help explain your results.
Hmmm... I'll take a look at the resistor tomorrow to see if I overlooked something and post the results.

14. The resistance of your wire is also a function of length and diameter, you are not taking that into account! Bresides its an alloy not a composite.

15. Originally Posted by fizzlooney
The resistance of your wire is also a function of length and diameter, you are not taking that into account!
Pardon? Please explain. I have to use the length and diameter (albeit area) to find the resistivity from the resistance.

Originally Posted by fizzlooney
Bresides its an alloy not a composite.
How does this affect the results?

16. So it turns out William McCormick was right. When silver and nickel are combined you get a resistivity that isn't within the resistivity of the two metals. Silver and Nickel are not very commonly merged,and therefore, finding a table for the metal is near impossible.

Thank you all for your input. I appreciate it.

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