Question for the "electrical" Guru's

[rockfla]

Why does polarity matter to some Volt meters/testers....For example, why will some testers test 6.3V on a V8 system going from black to negative and red to positive ie ground to ground, hot to hot, but not the other way around? Its the same voltage either way, right?

I have a digital Power Probe single wire voltage test probe and I can touch the alligator clip end on the negative terminal of the battery and the probe/meter end on the positive and get 6.3V reading....Do the reverse and "nothing", a blank screen. OR I can connect the ground clip to the ground strap on the firewall to the motor and try and test the voltage at the coil with it and get NO reading, blank screen. WHY? The ground strap to the battery AND the ground strap to the motor are both grounded off the firewall.

[wamnram]

Without knowing what probe you have, I'll guess: Your probe is polarity sensitive, and since you said it has a digital display, when polarity is reversed it stays blank. Since it is single wire, there is no internal "switch" to allow the solid-state electronics to work on reverse polarity.

[rockfla]

Originally Posted by wamnram

Without knowing what probe you have, I'll guess: Your probe is polarity sensitive, and since you said it has a digital display, when polarity is reversed it stays blank. Since it is single wire, there is no internal "switch" to allow the solid-state electronics to work on reverse polarity.

Which takes me back to my initial question: Why does "polarity" matter??? You get 6.3V either positive ground or negative ground and Volts are Volts SO why does polarity matter??? Enough so that there is a "need" of an internal switch OR posed a different way, why does my multi meter read 6.3V when black is on negative and red is on positive and will read - 6.3V when black is to positive and red is to negative???

[wamnram]

Unfortunately, Volts are not Volts. If you reverse the polarity, you go from Positive 6.3 volts to negative 6.3 volts. This causes the current to flow in the opposite direction. (Positive current flow to negative current flow). Digital circuitry does not like that.

[wamnram]

Also, Meters and Probes come in all kinds of varieties and capabilities. Most modern "Multi-meters" have internal circuitry designed to work on either positive or negative polarity. Old Vacuum tube meters (VTVM) required you to physically turn a switch to change the probe polarity. It all depends on the capability of your probe or meter.

[rockfla]

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Okay, I am not trying to be obstinate , whats the difference from a + 6.3V and a - 6.3V other than the flow SO again....why is the flow any different whether is flows positive to negative or Vice Versa??? How is one direction of flow a different flow over the other direction of flow when the outcome is the same???? It is basically a circular flow and where the two meet is the "action" why does it matter to a measuring instrument??? Is it one of those "mysteries O life" things???

[wamnram]

Please don't feel bad about asking questions, that's how we learn. Yes, very simply put, flow is flow. We don't care if river water flows from mountain to ocean or ocean to mountain, it's still just flow. The problem is that some instruments have been designed to measure the flow in one direction only. I have to run some errands so will be gone for awhile. We can discuss more later if you wish or maybe someone else can chime in.

[petehoovie]

Direct Current Circuit or DC Circuit is a closed electrical circuit in which the flow of electricity is in one direction. DC Circuit has a DC Power Supply which produces Direct Current in the circuit. As opposed to alternating current, Direct Current has a fixed magnitude and flows in one direction only - negative to +positive

[34fordy]

My Fluke 78 meter reads DC current plus or minus depending on where I place the probes. On AC current it reads the same both ways, not plus or minus as it is alternating.

[JayChicago]

Originally Posted by rockfla

SO again....why is the flow any different whether is flows positive to negative or Vice Versa??? How is one direction of flow a different flow over the other direction of flow when the outcome is the same????

Which way the electrons are flowing DOES matter to some electrical devices. But I don't understand electronics enough to explain this.

On our cars, ignition coils are polarity sensitive. But just about everything else is not. How about the horns? I dunno.

I think I read that solenoids don't care how they are hooked up. I think that is strange. It's an electric magnet. Why would it not be polarity sensitive? Why would it not pull the other way?

[rockfla]

Originally Posted by JayChicago

Which way the electrons are flowing DOES matter to some electrical devices. But I don't understand electronics enough to explain this.

On our cars, ignition coils are polarity sensitive. But just about everything else is not. How about the horns? I dunno.

I think I read that solenoids don't care how they are hooked up. I think that is strange. It's an electric magnet. Why would it not be polarity sensitive? Why would it not pull the other way?

Jay
You are on my same thought path as IF polarity matters, then why not to light bulbs, starters??

IF it polarity can flow two ways THEN why not make all electronic test devices work both ways???

It's the whole VHS Beta argument it seems!!!!

[Lawson Cox]

RELAX fellows. Just GO WITH THE FLOW" lol

[34fordy]

Originally Posted by Lawson Cox

RELAX fellows. Just GO WITH THE FLOW" lol

You're a little stinker counselor ! LOL

[wamnram]

The original poster asked: Why does polarity matter to some Volt meters/testers?

Answer: Because that is how some of them are designed. I believe specifically the probe he was using. Also, positive voltage is not the same as negative voltage.

34Fordy stated: My Fluke 78 meter reads DC current plus or minus depending on where I place the probes. On AC current it reads the same both ways, not plus or minus as it is alternating.

Reply: Because your Fluke is designed that way.

JayChicago: I think I read that solenoids don't care how they are hooked up. I think that is strange. It's an electric magnet. Why would it not be polarity sensitive? Why would it not pull the other way?
Today 02:21 PM,

Answer: Because a solenoid creates a magnetic field that draws an iron rod into the field. It does not matter the polarity of the field because the rod is not polarized, it's just a piece of iron.

Rockfla: IF it polarity can flow two ways THEN why not make all electronic test devices work both ways???

Reply: Nowadays, almost all electronic devices, especially meters, are designed and built to work both ways. The older a device is, the less this is so.

Hopefully this answers some questions without going too deep into electrical/magnetic theory. I don't want every one's eyes to roll up into their heads while muttering WTH? (Lawson, you da man!)

[rockfla]

Thank you Wayne.....SO....the still hanging question, on the larger view....IS why Henry Ford felt the need to go the opposite of what seems to be the "norm" by today's standard? What was his perceived advantage to the positive ground system?? Again, it seems as IF he was a Beta-Max man in a VHS world in my simplistic thinking brain cell and simplistic analogy!!!

[tubman]

For 6 volts, it's not "the opposite of what seems to be the 'norm' by today's standard". Most 6 volt systems were positive ground; Mopar, GMC, Studebaker, and others. Positive ground does not promote galvanic corrosion as much as negative ground.

[terry]

Then why have all cars gone to neg ground nowdays?

[tubman]

According to what I have read, positive ground was common up until about 1960, and was changed to negative ground with the move to AC generators (alternators). The reason stated was that the early rectifiers used in the alternators of the day would only work with negative ground. Once GM made it a standard, it stuck.