# how to measure the resistance of a Copper Wire?

Joined Feb 2, 2018
1
I need to measure the resistance of a copper wire.i have calculated the resistance value theoretically.I need to prove it practically.
Is there any method available to find the resistance?
I have tried using Multimeter.but the value is not read coz the wire has the resistance in milliohms..

#### kubeek

Joined Sep 20, 2005
5,795
What is the wire thickness and length? You would have to run some high current through it, measure the voltage drop and calculate the resistance.

#### ericgibbs

Joined Jan 29, 2010
18,859
hi Hari,
Welcome to AAC.
One method is to use a Wheatstone bridge and a Instrumentation Amp or a very sensitive DVM.
E

#### DickCappels

Joined Aug 21, 2008
10,185
Expanding on kubeek's post, I have made similar measurements by placing a low value resistor (0.1 ohm or so) in series with the wire and adjusting the voltage applied to the series circuit until I obtained a reading of the voltage droop across the wire on my DVM. Measuring the voltage drop across the series resistor yielded the current. The resistance of the wire is Vwire/I.

#### RichardO

Joined May 4, 2013
2,270
I normally put a resistor of a large value (compared to the wire resistance) in series with the wire. This resistor is never less than a few ohms. I set the power supply voltage to give 1 amp of current as calculated by the resistor value. This usually requires a high wattage resistor. The measurements only take a few seconds so a lower wattage resistor can be used as long as it does not get too hot to touch.

When I measure the voltage across the wire, I do it right next to where the connections are made -- on the wire -- never on the alligator clips.

#### strantor

Joined Oct 3, 2010
6,798
If you are only measuring the resistance of a single wire, use the trick already explained (pass current, measure voltage).
If this is something you will be doing quite often, it might make sense to buy a milliohm meter. I have this one and despite being cheap china, I have found it to be repeatably accurate.

#### WBahn

Joined Mar 31, 2012
30,066
Do you need to find the resistance of a particular piece of wire? Or just confirm the validity of the calculated results in general? If the latter and if you can, use a very, very long piece of wire -- like perhaps an entire spool -- and measure the resistance of that.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
So, you measure the current through the device and the voltage across any known length. You have to use different meters because of the usual ammeter series resistance not the same meter with two different functions.
A resistor in series will function as an ammeter. You don;t want to heat the wire.

The ampacity tables will give you the max current. Now, you just have to juggle the power supplies you have, the series resistance and meter resolution to get the accuracy you want without heating up the wire.

So, the voltage across the series resistor divided by its resistance gives you the current. 1 Amp, 0.1 A etc. are good numbers to use. Adjust for that current. Temperature will case these to change if you manage to heat the wire.

You don't want to be measuring uV or even pA. So aim for around 100 mV and say 1/10 to 1/100 of the rated ampacity of the cable.

Aside: R=pL/A where the p is Rho, l is the length and A is the cross-sectional area. Rho is a material property in units of resistance-distance; e.g. ohm-cm The formulas used by electricians, have in it an inherent round trip. You tun a pair of wires 40 feet. 40' is used in their formula, but 80' of wire use the fundamental formula.
They are interested in the voltage drop in %.

#### dl324

Joined Mar 30, 2015
16,933
I have tried using Multimeter.but the value is not read coz the wire has the resistance in milliohms..
Milliohms per what length?

Get a thousand foot spool of the wire and measure it. If resistance is still too small, use a Wheatstone bridge.