There are 4, 5 and 6 wire ohms techniques too:

https://www.tek.com/document/whitep...measurements-using-6-wire-ohms-measurement-te

 

Temperature would have some effect, though probably small; mainly, the ZERO adjustment is needed to compensate for changes in battery voltage.

I suspect you are thinking about the old VOMs and VTVMs. No?

 

The fluke 8012a, has a ohms zero function: https://www.matsolutions.com/Portal...ke/8010A/8010A Instruction Manual.pdf#page=17

 

I suspect you are thinking about the old VOMs and VTVMs. No?

Correct. All of the old analog VOMs I've had, had that adjustment.

 

Wouldn't one of those things be the temperature of the meter movement?

The temperature of the meter movement is one factor, but the biggest was generally the state of charge of the battery. The basic measurement is nothing more than responding to the current that flows with the battery in series with a range resistance and the unknown resistance. That's why the scale is nonlinear and why the lower the resistance the more the needle deflection. The zero is when maximum current if flowing with just the range resistor in the circuit, and that current depends directly on the actual voltage of the battery.

 

The temperature of the meter movement is one factor, but the biggest was generally the state of charge of the battery. The basic measurement is nothing more than responding to the current that flows with the battery in series with a range resistance and the unknown resistance. That's why the scale is nonlinear and why the lower the resistance the more the needle deflection. The zero is when maximum current if flowing with just the range resistor in the circuit, and that current depends directly on the actual voltage of the battery.

I have used VTVMs that plugged into AC that had similar adjustment. What was the deal there? Allowing for issues in the power supply?

 

I had a couple of VTVMs. one was ma RCA Senior VoltOhmyst what did not strike me as being strange at the time (but that I now question) is that in addition to plugging into the AC outlet for power they also had a D cell for the ohmmeter function.

 

I have used VTVMs that plugged into AC that had similar adjustment. What was the deal there? Allowing for issues in the power supply?

That, plus the ohms battery issue, plus tube drift with age.

 

I had a couple of VTVMs. one was ma RCA Senior VoltOhmyst what did not strike me as being strange at the time (but that I now question) is that in addition to plugging into the AC outlet for power they also had a D cell for the ohmmeter function.

Humm now that you mention it I need to think back. Maybe those old VTVMs did have a battery for ohms??? I think the last time I used one was way back in high school. That was a LONG time ago!

 

Good point, as I recall in the VTVMs there was a 12AU7 or similar connected as a differential pair with the meter movement indicating the difference in plate currents. That would be pretty sensitive to warm-up drift and aging.

 

Granted I have a cheap DMM. When I short the leads and measure in the lowest resistance scale, I am reading 2.6 ohms. I assume this is the resistance of the leads.

Even in better meters, leads have to have at least some resistance correct? How is that accounted for? Is there a way to zero them? Or do you just need to use leads designed for use of that particular DMM.

Seems a bit high - my old and grotty DMM leads are nearer 0.2R.

Probe tips can get tarnished, and so can the ends that plug into the meter - my meter is also old and grotty so I sometimes have to swivel the range switch a few times to settle the reading.

In extreme old and grottiness - the wire strands can fatigue and leave you less effective cross sectional area. Get hold of the ends and stretch it, if it snaps easily - it needed sorting out anyway. most often it will snap a very short distance from the probe, some probe tips unscrew and you can re solder the cleaned up lead. Usually the plugs into the meter are moulded for safety reasons - best not to try bodging them.