# What would cause an ohm meter to read .4 ohms when the leads are touching, but 0 when reading a short in a circuit?

I was talking to someone who does a lot of laptop board repair work and he was commenting that when he touches the leads of his meter together in OHM mode, he gets a reading of about .4 ohms ... which is not all that big a deal ... but then when he's checking a circuit for a short, he can get readings from the meter as low as 0 ...

I'm curious as to how that can happen?

• He's probably not digging the probes into each other when he touches them together like he does when testing the board. Apr 13, 2022 at 13:25
• Dirty nickel plating on probe surface might do that while the tips could have a better conductor with sufficient pressure. Apr 13, 2022 at 13:40
• contact resistance is very variable, up to 1 ohm is not uncommon, unless you have gold on gold. Apr 13, 2022 at 14:00

This is likely due to the sharp probe tips making a better electrical connection than the side of the probes do. (The probe sides may have a thin layer of oxidation or other contamination.)

• That's an excellent point! Thank you Apr 13, 2022 at 14:37

If there are any residual voltages on the circuit board that appear across the component being tested, this will upset the meter reading. This can happen because of charged capacitors. For instance, if the ohm meter's open-circuit voltage is (say) 1 volt and you try and measure the resistance of a 10 kΩ resistor that has a residual voltage of 1 volt across it, then the meter will read infinite ohms because no current can flow.

And, if you reversed the probes the meter might read a negative resistance. Be careful about measuring resistances on PCBAs that still have charge remaining on capacitors. Here's a situation that would show exactly 0 Ω even though a physical 10 Ω resistor is supposedly being measured: -

simulate this circuit – Schematic created using CircuitLab

So, if you want to measure R1 above, first try measuring voltage, and see that 0 volts is indicated. Anything else would is a red-light warning if you then tried to measure resistance.

To measure low resistances smaller than about 1 Ohm a 4 wire measurement method delivers better results. Two wires for the measurement current and two wires for the voltage drop over the unknown resistor to be measured.

The result of a 4 wire resistance measurement is not influenced by the resistance of leads and probe tips.

The voltage probes should be applicated very close to the resistor under test, the probes for the current outside the voltage probes with a little more distance.