James,

There are a pair of SCRs, one for the positive half cycle, and one for the negative half cycle.

I think of the basic graph from my electricity textbook:
[Linked Image from ecmweb.com]
This shows what a standard $8.00 600 Watt incandescent only dimmer waveform looks like. . .kind of. It's actually cleaned up a bit with a nice square turn on "rise of voltage" that smoothly connects to the sine wave.

T, T, Ness, your #2 question goes to the heart of what actually happens when the dimmer switches ON each half cycle. The voltage doesn't go instantly on, rather it ramps up. The straight up and down line actually leans towards the sine wave, so the voltage that was at zero increases at a quick, but predictable rate. When the voltage gets up to the sine wave, the voltage should stop there, but because this is a real world circuit, not a theoretical circuit the voltage overshoots, falls back to the sine wave, undershoots, returns to the sine wave, overshoots. . .well, it settles to the sine wave, but it takes extra circuitry to get it to do it with very little oscillation.

Compared to the sinewave, this over - undershoot oscillation dies out quickly, but it does it every time the dimmer switches on each half cycle.

Rise Time is the amount of time that the voltages ramps up to the sinewave.

For Question #1, the effect of the full wave dimmer is to raise and lower the voltage of the entire sinewave keeping the sinewave intact, rather that keeping it off for a part of each cycle. Noticably absent is the sudden switch on of the rapidly rising ramp of voltage.

Now, that rapid rise time turn on is a curious animal, electrically. It is, in effect, a collection of high frequency sinewaves, much higher than the 60 Hz power sinewave. Even though the rise time of the "turn on" is short, it can have destructive effects on magnetic loads or electronic loads.

Al Hildenbrand