Resistivity of Nichrome experiment

[ABarrows]

Every year we do an experiment to find the resistivity of Nichrome wire, and every year the result is the same: 5 x 10^-7 instead of 1 x 10^-6. For the life of me I haven't been able to track down why it's a factor of 2 off.

We use a Wheatstone bridge that has a 1 m length Nichrome wire stretched over a meter stick. A Heathkit power supply (either model SP-2710, IP-2711, or SP-2720) feeds the current through a Fluke 75 multimeter set as an ammeter; a patch cord from the meter's COM terminal is clamped via an alligator clip to a sliding contact that moves along the meter stick, and the supply's negative terminal connects to the plug-in at the zero end of the wire. A second Fluke 75 multimeter serves as a voltmeter, with its patch cords accordingly plugged into those patch cords previously mentioned.

They start at the 5 cm mark and work out to the 80 cm mark in 5 cm increments, measuring the voltages with a constant current of 0.5 A. The instructions say the wire's diameter is about 0.5 mm--I got 0.515 mm when I checked it with a micrometer, so that's not the problem. If I measure the resistance of the wire with a multimeter directly, I get about 2 ohms; this is exactly what they get in the first part of the experiment when they use a 75 cm length and measure corresponding voltages for currents from 0.05 A to 0.5 A in 0.05 A increments. I checked the patch cords and found they do not lend any appreciable resistance to the circuit (they all measured 0 ohms with the multimeter when connected together).

I'm out of ideas as to what else to check to track down the discrepancy. Any suggestions?

 

[technician]

using your value of 2ohms and 0.75m length and 0.5mm diameter I also get 5x10^-7 !
My textbook gives the value for nichrome of 'about' 1x10^-6 so there may be variation between samples !... grasping at straws.
Are you certain it is nichrome?

 

[ABarrows]

I considered that it just might have a low value, but everything I've been able to find says Nichrome's resistivity varies from 1.0 x 10^-6 to 1.5 X 10^-6, so I doubt there's a composition variety with 0.5 x 10^-6 as its resistivity. And no, I'm not 100% certain it's Nichrome, but I have no reason not to take the other professor's word that it is (wish he had kept the paperwork that came with the bridges when he bought them).

 

[ABarrows]

Well, we figured it out (just in case anyone is curious). Turns out it is Manganin, not Nichrome--we used an ohmmeter to measure actual resistances for various lengths, applying the R=ρL/A relationship directly without running current through it (as the students do for the experiment), and we got the same resistivity as always. A Google search then led us to Manganin as the metal with a similar resistivity value (4.82 x 10^-7 Ωm), and Manganin's wikipedia page says Wheatstone bridges are often made with this metal. Problem finally solved. Huzzah!