My goodness. I haven't seen a thread with so many misconceptions in it for a while. Let's see what we can clear up.
#1. Residental AC
In a normal residental home, you have an available 240 volts, with 4 different leads. You have 2 hots, both with ~120v available. I say
~120v because in actuality, you can see anywhere from about 100 to 130 volts at any given moment on each lead. Now, the 2 hots are out of phase with each other by one wavelength. This means that 2 hots will give you ~240v (the wavelengths combine) or any single hot and the neutral will give you ~120v.
Now, the neutral is just that, neutral. IT IS NOT GROUND. Now, unless something is wrong witht he wiring in the home, there should be zero volts on this line. However, this line can be carying a good amount of amperage, but if the house was wired correctly, then the amperage on the line should be minimal (generaly less than 20 amps). If you look at your breaker pannel, you will notice 2 rows of breakers (in the vast majority of boxes) If you go down one side of the panel, the first breaker will be on one 120v line, the second will be on the other. (they are hooked to what are called Bus Bars)
Basicly, every other breaker is on the opposite line. This means that when the amperage is combined on the neutral, the amperage cancels out because theoreticly 1/2 of the amperage daw is exactly 180 degrees out of phase with the other half. This is why, generaly, neutral wires are 1/2 the gauge thickess (higher numericly) then the hots that supply them. (most residental is 2 4 gauge hots and an 8 gauge neutral return)
The ground line is just that, ground. There is either a 5' rod (or a few depending on soil conditions) hammered into the ground by your fuse panel, or a line from the ground bus bar to the water main (generally only if it is copper, but I have seen some on galvinized lines). If you have a line to the water main, then you should also have a jumper across the water meter, as the meter can stop the current flow under some conditions.
Now, a while ago, someone figured that having a voltage on the neutral was a bad thing (duh!) and the neutral bar is now required to be tied to the ground bar. This can be done a multitude of ways, but usually, there is an 8 gauge wire between the 2. Sometimes, in newer panels, and almost all J-Pannels, the neutral and ground bar are the same bar. What this does is cause a breaker trip if voltage is on the neutral bar. Theoreticly, any voltage on the neutral bar will pull an infinate amount of amps as there is no resistance, which will cause the offencing circit breaker to trop or the fuse to blow.
As a side effect to this, it is now required to ground the ductwork (hvac) in residental homes, as that was, in some instances, capable of having some voltage on it because of f'ed up wiring. Without the ground, there was no where for it to go. Well, not until you stepped onto the metal grate in the floor and got an owie. Ok, so that's residental wiring as applies to this discussion. Sorry it was so long.
#2.
Automotive and dry cell batteries (DC)
First, AC works differently than DC. DC only goes in one direction, from positive to negitive right? Well, actually not. The electrons actually go from the negative side and fill in 'holes' in the positive side. In this instance, you need 2 things for current to flow. You need the electons, and you need the holes. Since you need both, only a hookup from the positive terminal of one battery, and the negitive terminal of the same battery will you get current flow. If you use one battery's positive and another batterys negitive, you will get exacly zero current flow. Now, as far as voltage is concerned, there is a 1.5v (or 12.6v for automotive) potential difference, but it can not be measured or used in any way. It is only there in a mathmaticl sense. Try it yourself on 2 different batteries. If you don't believe me, I have 2 fully charged automotive bats in my garage, I can take pics/
#3.
Electrical meters
I am only going to go into volt meters for now so I can keep it simple. The way that most meters work is aparently misunderstood, even by some EE majors. Which really concerns me, and lays waste to my hope that the human race is getting smarter. Anyway...
Most volt meters do not mesure voltage. Let me repeat that, because it bares repeating. Volt meters do NOT measure voltage. In a perfect world, a voltmeter will have infinate resistance. However, I can not go to Radio Shack and order a resister with infinate ohms. Even special order! (what would the bands look like anyway? plaid?)
What is done is an extreamly high resistance resister is chosen (generally in the millions of ohms) and the AMPERAGE flowing though the COMPLETED circuit is how the voltage is computed. Because the meter actually measures AMPERAGE, the circut can not be energized, as the amperage flowing through the circuit can mess with the internal computations. That is why, on cheap meters, you get garbage when you measure voltage on a energized circuit.
As for the resister, (or resister block) the resister value is high for 2 reasons. The first is to limit the current in the circut as much as possible. More amperage means more heat, which means a larger and more expensive unit. The second is so that the meter can be as acutate as possible while still being made cheaply. Resisters generaly are within 5, 10 or 20% of their listed value. If you are measureing something with 1.5v and a resistance of a few million, being off by 5% on the few million side will result in a very small change in the reading.
A good analogy to the volt meter is a horsepower rating on engines. Engines to not make horsepower. engines make torque, or twisting force. Horsepower is just antoher way of measuring power output. Horsepower is computed by taking the torque, miltiplying it by the RPM the engine is at, and dividing by a contstant value, which is 5252. This means, given the same amount of torque, a higher reving engine will have more horsepower.
Back tot he volt meter. Since a connection between the positive side of a battery and the neutral or ground line on a home, or the negative of another battery for that mater, will not cause current to flow, then the volt meter can not read it, because as I said earlier, it measures AMPERAGE. Also, as I said, there is actually voltage potental there, it is just not measureable with a voltmeter.
#4. Conclusions
Everyone needs to shut the hell up and stop bashing other people and posters. In this entire thread, I see very few actual correct answers, only people trying to make their e-penis's bigger. If you are going to have a discusstion, great, have one. But don't start beinging insults into the mix, expecialy when you are insulting someone for being wrong, while you yourself is in the wrong. Now, this is not pointed at anyone in perticular, just OT in general. Ok, rant over. Enjoy the rest of the, hopefully civil, thread.
BTW Eli: I hope you enjoyed this really long post
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