question on project "Which Materials are Best Conductor

[steve-o]

We are using the Science Buddies project entitled "Which Materials are the Best Conductors?". See:

https://www.sciencebuddies.org/mentorin ... ?from=Home

The experiment involves measuring the resistance of various materials. The project describes a circuit to do the measurement consisting of a 6V battery and and ohm-meter. But my question is, why do you need to 6V battery? Why can't you just use the ohm-meter to measure the resistance? After all, that is what an ohm-meter does. Thanks.

[smarti]

Is this the steve-o from colton?

[steve-o]

no

[deleted-71576]

I'm not sure it would be necessary either. There are some materials that change resistance with current applied, but they are few and far between, and not important for your experiment.

I would think it would be just fine to go ahead and measure the resistance without the battery.

Any of the EE experts, if I'm wrong, please correct me.

[ghariman]

Hi,

I agree that you do NOT need a 6V battery as the ohm-meter applies a small current through your material and measures the voltage:
http://www.doctronics.co.uk/meter.htm

If you want to have second data to confirm that your ohm-meter is working correctly (sanity check) then you could re-take the resistance measurement by applying 6V across your material and measuring the current and then calculating the resistance. See how well your ohm-meter works for different types of material. However 6V across a good conductor would heat up the conductor.

[steve-o]

Thanks so much for your answers. So, my followup question is, what effect does putting the battery in the circuit have? If you have the ohm-meter set up to measure resistance, and then you have the leads across not just a test material, but the test material in series with the 6V battery, what do you end up measuring? What is the resistance of a battery? (Note the experiment as described does not even tell you which polarity of the battery you should use.)

[deleted-71588]

You have found yet another science buddies experiment that has a major error in it.

DO NOT USE ANY EXTERNAL VOLTAGE SOURCE when measuring resistance with any Ohm meter.

There are several ways to measure resistance. Most multi-function and DVM based meters built in the last 30 years that have Ohm scales have an internal battery and zener diode to provide an accurate voltage or current source whose accuracy will be severely affected by any external voltage source. Additionally, if the external voltage source exceeds the break down voltage of any of the meter circuits, permanent damage to the meter may result.

[deleted-71588]

Sorry steve-o, I didn't read you last post before replying. Without knowing precisely what the internal design of your meter is, predicting its precise behavior with the 6 volt battery in series is impossible.

If the meter design is such that the polarity of say an internal 5 volt source is in series with the 6 volt external source, the current flow will be 11/5 times what the circuit expects and result in the reading might be 5/11 of the proper value. If the polarity is reversed, the instead of +5 volts you will get +5 - 6 = -1 volts, the current will be flowing in the opposite direction and all bets are off on what happens to a digital meter. An analog version would likely peg the needle against the mechanical stop at the 0 end of the scale possibly bending it in the process.

[steve-o]

Thank you, Craig!