If you take the common lead (black) to a (-) dc supply source and the red lead to a + DC supply source, will the voltage be negative or positive?

positive

It doesn't matter whether one or the other terminal you connect to is grounded or not, positive or negative with respect to ground.

The reading you get on the meter will depend entirely upon the potential difference between the two leads. If the "+" lead is more positive than the "-" you'll get a plus reading. If the "-" lead is more positive than the "+" you'll get a minus reading.

You could connect the positive test lead to a point which is at +6V to ground, the negative lead to a point which is at +9V to ground. Even though both test points are positive with respect to ground, the meter will give you a reading of -3V.

If you are using a digital meter it does not matter. But look for the plus or minus symbol on the display. Then you can reverse the leads. But the voltage value will not change.
I think with an analog meter your pointer will try to go negative, thus providing no reading. This is when you need to put the right meter lead on the right conductor?????
Anyone else?

It will. Depending upon the range selected and the applied reverse voltage it will do anything from moving slightly to the left of the zero mark to slamming hard into the left-hand stop (and possibly causing damage).

You can get center-zero movements intended to allow either polarity to be applied (the ammeter in your car is a good example).

The VTVM (vacuum tube voltmeter) or its modern transistor equivalent often has a polarity reversing switch so that negative values can be read with the cable shield left connected to a grounding point.

Man, I remember one sure fire way to cause the veins in the vocational school instructor's head to pop out was to reverse the leads on the schools analog multimeters while doing dc testing since he had repeatedly warned that it would damage the meters.

There are lots of things that will blow up an analog meter and a digital meter will shrug off.
I let the smoke out of a meter once switching from DC to AC, transiting the milliamp range.

Come on guys: Black (common) lead to -ve side of cell (DC source), red lead to +ve side of cell = POSITIVE reading on an analog meter (needle moves to the right)

There is one, not so obvious, thing to watch out for on DVMs, VTVMs, and VOMs. Some are reversed on resistance or diode checks. All of my Flukes have been correct, or should I say intuitive, with - on the black lead for diode checks. My Sabtronics DVM at home has - on the red lead on all resistance scales. I need to keep that in mind because I often use it to check LEDs on the 100 ohm X10 scale. I try not to reverse bias LEDs because they typically have a very low reverse breakdown voltage.
Joe

Most of the older, conventional-style VOMs were wired in the way that Joe describes, so that polarity from the internal battery was the reverse of what you might expect on ohms ranges. It simplified the switching to do it that way.



The Avo 8 series meters (among others) actually use the positive terminal as the common for all ranges. The negative test lead is then connected to one of three terminals for switched ranges, 2500V A.C., or 2500V D.C. (it's not "negative" as such on A.C., of course).

Paul,
I have an old meter here, that was one of the first to use Field-Effect-Transistors (FET's).
It gives a centre zero option, although I've never used it before, it also has a polarity switch on it too.
It says on the scale that it will measure up to 3000VAC, personally I would never use it to that level of voltage.
However,
One thing my younger brother learned in a hurry when I first started my time as an Electrician, don't plug your older brother's meter into the 230V socket and expect to measure how fast the amps are coming out of the socket on the resistance range.
There was a shower of sparks and a blown fuse at the switchboard and a useless meter.

Most VTVMs or FET equivalents allow that. The polarity reversal is needed so that you can measure voltages positive or negative with respect to the chassis with the shield of the cable/probe on the latter (e.g. negative A.G.C. or grid bias voltages, or most voltages on those older transistor units based around PNP transistors). There's normally enough range provided on the zero adjustment (the electrical one, not the mechanical one) to bring the meter up to center zero (with suitable scale provided).

I still have an old Leader VTVM (mid 1960s) which I use for radio work.

And of course, when making that chassis connection you need to make sure it isn't a live chassis, odervize ist spitz und sparken mit poppenfusen!