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You don't seem to understand how dc current flows...
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You are correct, aifam55 does not. Unfortunately, neither do you.

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There cannot be negative voltage with dc current because the current flows only from one polarity to another. (you can have between 0V and xV)
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Actually, no. By definition of the electron, there is no such thing as a positive voltage. ALL voltages are just more or less negative than others. We describe them in many different ways, but that does not change the physics. If you are going to make pronouncements about other's understandings, best to check your own first. Glass houses.

It is important to remember that positive, negative, up down, north, south - all of these are completely arbitrary terms with no "absolute" basis in physical reality. We call a mobile sub-atomic particle an "electron" because some ancient Greek thought it up one day. And the charge of an electron is "negative" for the same reason. Unfortunately, "negative" has, well, negative connotations in other contexts. We rank numbers with positive values above negative values when we write them in a list, and we design circuits such that it looks like the energy comes from the "positive" power source terminal, flows through the electronic components, and returns to the "negative" terminal. We even draw schematics with the positive voltages toward the top of the page and the negative voltages toward the bottom because water flows downhill (honest-to-god that's where this comes from). All of this is arbitrary, and backwards from the original arbitrary designations of electrons having a "negative" charge. Note that for the first 10 to 15 years of transistor radio design, and many automobiles of the 40's, 50's, and 60's, the positive battery terminal was the system ground.

Victim2 is correct, electrons flow from negative to positive. This is why the phone system runs on *negative* 48 volts. Bell knew very little about electricity, but he did know that electrons had a negative charge, so he designed the phone system with the correct orientation. Unfortunately, the rest of the world took a more "positive" view of physics, and now we have this confusion in electrical engineering between "classical" and "conventional" electron flow. This might seem like pissy little semantics, but it is critical to understanding how things really work.

For something like a battery-powered zapper, there is no Ground in the classis sense. Ground, as an electrical engineering structure, has very specific meanings developed and refined over centuries. First of all it is the electrical potential of dirt, earth, the planet. Ground is an infinite source and sink of electrons. Depending on where you live, it might be a fire code requirement that there be a ground rod driven into the earth outside your house and connected to the fuse box. That is Ground with a really big G. Electronic systems that are isolated from earth ground by the transformer in a power supply, or by being battery-powered, do not have a physical relationship with Earth Ground. Still, the circuit functions because there is an electrical potential that is the agreed-upon reference for design, and this reference potential is called the circuit ground or system ground. On schematics it has a symbol that is different from Earth Ground. With respect to Earth Ground it can be positive, negative, or arbitratily floating - none of this affects how the circuit functions. Electrons move from the more-negative to the more-positive parts, we describe energy flow as going from the more-positive to the more-negative parts, and that's just the way the language of electronics has developed.

OK, I've wandered off a bit.

If you sit outside in a plastic chair with rubber-soled sandals on your feet, you are very effectively insulated and isolated from Earth Ground. Hook up your zapper, attach/grab the terminals, and it will work as usual. Now, run a small wire from the positive battery terminal to a screwdriver stuck into the dirt at your feet. This places the positive terminal at Earth Ground, and the negative terminal now is at -9 volts with respect to Earth Ground. Guess what - the zapper continues to work exactly as before. As long as you are insulated from Earth Ground except for a single point of contact, there is no conflict in potentials. This is why it is important to remember that all polarity designations are arbitrary. What is important and non-arbitrary is how the physical relationships and connections affect the movement of energy. Part of this thread is about how organisms move when influenced by an electric field. Absolutely true, but don't confuse things by tossing around terms that don't apply.

ak