Don't listen to electricians on this stuff. They know how to wire you house, but it is very rare that they actually know a lot about how electricity works or how to describe it.
This is how AC systems actually look. Yes, as Speedskater says it is really only a single phase. However the phase gets split (center tap) so that you can have 120v instead of 240v. This gives you two effective poles that are "hot" (Line). These are the alternating spots for breakers that Ron mentioned.
Neither is positive and neither is negative on the poles. As you can see each pole has 360* of phase, and it flips between "negative & positive" every 180* there for there is "phase" because it is
alternating (AC) up and down at 60 times a second (60hz). The neutral as you can see by itself cannot complete a circuit by connecting to SG (safety ground, the one connected to ground rod), because the path doesn't cross any of the transformer windings (SG & neutral are connected). But for both Hot poles (Lines), you can see that if they touch the other Hot, or SG it makes a complete circuit that includes the winding of the transformer. In this picture there is a primary and secondary winding, and the CT splits the secondary winding in two.
Here's an example of how Neutral cannot make a path through any windings of the transformer to complete a circuit that has power:
Here's how a normal connection is made for 120v that will complete a circuit through the transformer:
Here's how the two Hot poles can make 240v completing a circuit through the entire phase of the transformer:
Stereophonic: What your picture shows is just a way to determine which socket hole is Line (Hot). You put one test probe on SG and then see which L or N gives you 120v. The one that does give 120v is Hot, and true Line. If the small blade on the socket, or the L marking on an IEC cord does give you 120v, then you have the polarity swapped at some point (L and N were swapped).
audio.bill: Your link actually has nothing to do with polarity or phase at all of the electrical system, and only to do with polarity of the equipment. What it describes is finding which input wire of your transformer has the least amount of capacitance to the enclosure. Sadly there isn't much you can do about the problem without getting into the amplifier. Yes you can fiddle by swapping wires in the cord, but what you do is change where the fuse is applied, and that is a bad idea. Hopefully the designing should know better... The correct fix is to swap the transformer leads to corresponding switch & fuse, and neutral.
Ron Resnick: Again there is no positive. What your electrician is wanting to say, were he to understand it much better, is that you should only use 1 pole (1 Hot side of the transformer 120v) for the whole stereo. This is a decent idea for a few reason. Here's a drawing where you can see only one pole is used for the sockets. Please note that in a real breaker box the poles are mixed every other.
What a real breaker box looks like, as you can see the poles alternate. This is because the part in the middle that rises up connects to breaks on left and right at the center of the breaker box.
Stereophonic: The plug style doesn't matter, in your country the transformer simple puts out a different voltage. You still have L, N, and SG. But I don't know if you have 2 poles per residence from a single phase. I'm sure you're on single phase, but couldn't speak for whether you have 1 or 2 poles. It may be different from country to country, but 1 pole looks to be a common thing in some other countries besides US.
