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 text book 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!