Measuring resistance accurately?

1. Jul 31, 2008

BioCore

1. The problem statement, all variables and given/known data
I was wondering what method is exactly more accurate. Would using an ohmeter, that is part of a multimeter set be more accurate than looking at the color code? My guess is that the ohmeter would be more accurate even though it is not a dedicated ohmeter, but I am not sure as to how to explain why it is.

2. Relevant equations

3. The attempt at a solution
My guess is that the ohmeter would be more accurate even though it is not a dedicated ohmeter, but I am not sure as to how to explain why it is.

Also, will the answer be different if we are discussing specifically apart low resistances and high resistances?

Last edited by a moderator: Jul 31, 2008
2. Jul 31, 2008

mgb_phys

Resistors come in a range of tolerances 5%,1%,01% so if you have a 5% resistor the color codes could legitimately be out by 5%.

An ohmeter (wether built into a multimeter or not) passes a fixed current through the resistor and measures the voltage across it. This answer of course depends on the accurcy of the fixed current and voltmeter.

As to your last question - what would happen if the resistance of the resistor approached the internal resistance of the voltmeter?

3. Jul 31, 2008

Redbelly98

Staff Emeritus
And it can be off by even more than that, if the resistor has ever been driven above it's power rating.

I was once a lab T.A. for an introductory electricity/magnetism course. One of the labs was to measure a resistor (I forget just how, might have been using a Wheatstone Bridge). I deliberately selected old, used resistors that did not match the color codes, measured with an ohmmeter with those measurements known only to me.

It was a real eye opener how many students altered their data to make the results agree with the color codes. Even one pair of lab partners, supposedly with the same data, coming up with different answers (one got the actual resistance, the other got the color-code value).

4. Jul 31, 2008

mgb_phys

To be really evil you substitute resistors made of wood - it's fun watching them swap power supplies, meters etc......

5. Jul 31, 2008

Redbelly98

Staff Emeritus
LOL. But seriously folks ...

Back to the original question. Just using the color codes is not, in my opinion, a measurement. It is akin to saying you measured the power of a lightbulb by reading the "100 Watts" printed on it. That's not measuring, but taking somebody else's word for it as to what the value is.

So: the more accurate measurement, in this case, is the method that is a measurement (using an ohmmeter), and not the method which isn't a measurement (reading the color codes).