How can I measure the conductivity of a copper rod?

I would like to perform an experiment to measure the conductivity of a copper rod. What device can I use to perform that experiment? Is there such a thing as a conductivity meter? All I found was an apparatus to measure the conductivity of solutions. Please note that I need to get results as accurate as possible.

• can't you just measure the resistance and then invert it 1/resistance? – kenny Jan 29 '12 at 16:09
• I think its possible, but with my calculations, the resistivity of the rod may be in the micro/milli range, which is not detectable using regular ohm meters. – ABA Jan 29 '12 at 16:14

Apart from the well known 4-wire measurement method there is another interesting method of measuring conductivity, especially useful for larger samples: you could call it a "0-wire" measurement beause it works without even touching the material:

Eddy current testing:
A coil is used to create an alternating magnetic field, that induces alternating eddy currents in the material (proportional to its conductivity). Those currents in turn create another alternating magnetic field that affects the impedance of the coil.

See e.g. this link or this pdf.

Very low resistance resistors need to be measured differently from how most ohmmeters work. This is because the resistance of the meter leads would be significant compared to the measured resistance.

The solution is "4 wire" measurement. Two wires are connected to the ends of the rod and are used to run a known current thru it. This may be a few Amps. Then make two more connections near the ends of the rod and measure the voltage accross them. The resistance of these wires won't matter since they have very little current thru them. The first set of wires may have more resistance than the copper rod. This means they drop more voltage than the rod, but that doesn't matter since you are measuring the voltage only accross the rod. Any extra voltage the power supply sees due to the connection wires is irrelevant as long as the total is low enough for the power supply to reliably supply the known current.

Given the current thru the copper rod and the voltage accross it, you can compute its resistance from Ohm's law.

• Would this type of mili-ohm meter succeed in giving correct values? link I noticed that it has the "4-wire" testing in its specifications. – ABA Jan 29 '12 at 17:05

There is an instrument that will read the low resistance of copper. It is often used in the power industry for measuring copper bus bar, permanent and portable grounds. It's called a "Ductor".

Not directly the conductivity, but with a multimeter you can measure the resistance of the rod (but probably it will be very low) and calculating the average section and length you can obtain the resistivity, that is the inverse of the conductivity.

To measure resistance accurately, you must use the 4-wires method (with the 2 added wires you can sense accurately the voltage across the rod, as they are not carrying current), to have the real voltage over the rod; a more sophisticated way, that works better if you have more than one rod, is creating a wheatstone bridge, that allows very accurate measurements. But you need very accurate resistances of nearly the same value. Moreover, good ohmmeters already embed a bridge circuits for measuring resistances.

Copper conductivity [S*m/mm²]: 58.5

That means that a wire 1 meter long and with a 1 mm² section, you can expect a resistance of $${1 \over 58.5} = 0.017 \Omega$$

Now, if your rod is larger and thicker it will be worse...take this into account.

• This is the point of the experiment. to derive the standard value like the tables. – ABA Jan 29 '12 at 16:48
• @clabacchio, Conductivity of copper depends (slightly) on any alloy content and how it's processed. How much it was worked to form the rod, and whether it was annealed afterwards, for example. So its not at all a bad experiment -- and you can't trust the standard value if its really critical to get a certain resistance in your design. – The Photon Jan 29 '12 at 17:00
• @ABA, Conversely, the measurement won't give a standard value to be used forever afterwards...if you change suppliers or use a different form (rolled copper on a pcb is different from bar stock, for example) of copper, you'll want to re-measure. – The Photon Jan 29 '12 at 17:02
• @ThePhoton Should I assume that different forms of copper will show higher resistivity than the standard value? – ABA Jan 30 '12 at 13:29
• @ABA I suspect that's correct, but I don't know for sure. Here's a list of alloys (ndt-ed.org/GeneralResources/MaterialProperties/ET/…) and only a couple have a tiny bit higher conductivity than "pure (annealed)" copper. – The Photon Jan 30 '12 at 16:41

That's really complicated... An ohmmeter clearly won't do. The copper rod is a very good conductor, and as pointed out earlier, its resistance will be out of the range of the meter. I'd suggest using a Wheatstone bridge, or better even a Kelvin bridge.