Discussion in 'General Electronics Chat' started by clarkk, Jul 18, 2013.

1. ### clarkk Thread Starter New Member

Jul 18, 2013
2
0
Sorry the title is worthless, i got an error every time so i kept making it smaller. So i am new to this and i and have confused myself. When testing voltage the multimeter reports the difference in voltage between the two points you test. So if i put the red probe on the positive terminal and then the black probe on the point where the negative cable connects to the ground i get a reading of 12.6. So to me this means the pos terminal holds 12.6V and the ground is 0 and the meter reads 12.6V because that is the difference between the two. So i reason that this does not necessarily mean that these two points are connected in a circuit because the meter doesnt know if they are in the same circuit it is just measuring the difference in voltage between the two points. So if this is true i could test between the pos terminal and a ground on a seperate circuit and it should give the same reading. But it didnt - i got a jumpy reading of 3.5 or something. Is my logic wrong?

Last edited by a moderator: Jul 18, 2013
2. ### LDC3 Active Member

Apr 27, 2013
920
161
There must be a continuous path from one lead to the other lead to get a valid reading. When you connected the read lead on the positive terminal and the black lead on the ground, there was a path from where you touched the ground to the negative terminal of the battery.

In the second case, unless the two grounds are connected, then the reading on the voltmeter is invalid.

3. ### Wendy Moderator

Mar 24, 2008
21,428
2,953
Ground is ground the world round, but not on vehicles.

4. ### #12 Expert

Nov 30, 2010
18,076
9,676
Strange as it might seem, two identical car batteries, well known for having zero and +12.6 volts each, have no knowledge of each other unless they are connected in some way. Each one is an ignorant little island of voltage with no reliable relationship to anything it is not explicitly connected to.

5. ### wayneh Expert

Sep 9, 2010
15,554
5,763
Another way of saying that is that what we call "ground" is usually just the lowest local voltage, not the global lowest voltage. On a battery it's the negative pole. In a vehicle it's that pole and, sort of, a lot of the chassis and such that is connected to it by ground straps. Those occasionally fail, though, and cause real havoc.

Other than earth ground, the commonly used term "ground" has no meaning when two points have no electrical connection. Ground on my flashlight's AA battery is not related to ground on my car's battery.

6. ### clarkk Thread Starter New Member

Jul 18, 2013
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Got it! Thanks for the help. I did some reading and the voltmeter creates a parallel circuit and has to have current flowing through it for it to take the measurement. That seems so obvious now, how else would it take the measurement without current from the circuit going through it. So when i took the reading between the two unrelated circuits there was in fact current going through the meter to be measured but since ground is simply the lowest local voltage i wouldnt expect ground on the unrelated circuit to read the same as the ground on the connected circuit.

7. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
521
Do not write this reasononing off, it is not so loopy as it sounds.

You cannot, of course, expect the ground (used in this case as a reference level or voltage) to be the same in unconnected circuits.

So a voltage with one value with reference to the ground in one circuit may well have a different value with reference to the ground in another.

But there may well be a voltage between them.

However you will not be able to measure this with the equipment you were using.

A special type of voltmeter is needed for this.

8. ### wayneh Expert

Sep 9, 2010
15,554
5,763
The water analogy is two ponds on a mountain. A normal voltmeter measures the depth of each pond. You need different equipment to measure the different elevations of the ponds on the mountain, even if they are connected by a trickling stream (a high-resistance electrical path).