As you said, slow and steady is the way to go. The reason some are using two elements, one larger and one smaller, is that it gives them much finer control of the heat into the pot. A power control on a 1500 watt element is going to have a lot coarser adjustment than the same control on a 250 watt element (think percents).
There are two subjects I haven't seen discussed here that I think might help understand what's going on. The first: Temperature is the measure of
average kinetic energy. This means for any mass, lets say of liquid(s), some few molecules will actually be a very low kinetic energy, even freezing, and some will be very high. The
average will be whatever temperature your thermometer reads (well, the average
near the thermometer, anyway. See the next paragraph). This is why liquids evaporate, even at very cold temperatures. Even water ice will lose some mass to vaporization (look up sublimation). The reason this is important is this: In a standing pot of room temperature water, say, a molecule gets enough energy (we won't go into HOW here..) to become vapor. Almost every time this happens, it will 'collide' with 'colder' molecules and become colder by energy transfer. If this molecule were at the surface of the water, there is a chance it could 'escape' and stay water vapor, but more than likely air molecules would cool it down, and back in the drink it goes. Some do manage to escape, and we get water vapor in the air (a.k.a. humidity!). The same thing happens in a liquid mixture, in all different proportions depending on physics, but the important thing to remember is that even in a perfect (impossible) mixture where everything is at the boiling point (Average Temperature) of alcohol, there is a 50/50 chance that an alcohol vapor molecule will hit something colder and become liquid again before escaping.
Sorry this is so long, but the second topic is just as important. Getting your whole still to the same exact average kinetic energy is physically impossible. It would be really nice if you could - get everything to the exact boiling point of alcohol and bingo! Perfect distillation! Unfortunately, we live in the real world. First: Evaporation removes energy from a liquid by converting it from kinetic (temperature) to potental energy (heat of vaporization). This means that the energy you are adding
isn't changing temperature (kinetic energy). Its changing liquid to vapor (potential energy), and the only way to measure that is by the amount of vapor. Second: Everywhere there's a temperature boundary (the walls of your still, the packing, the condenser...) there is a temperature gradient. Your heating element may be 250 deg. The inside wall of your pot will be a degree or more less (the thicker, the cooler) because the liquid is taking heat from it. As that heated liquid circulates, it heats more liquid, but becomes cooler by doing so. This plus vaporizaon makes a temperature gradient in the liquid itself. Those initial bubbles that form on the bottom of a pot before it boils don't look like much, but they are roiling heat transfer factories, with liquid constantly vaporizing and condensing at the bubble wall. All this is to say you can't get the same perfect temperature even in the liquid alone, forget the whole still.
If you're having trouble understanding why you can't control your still by temperature, this is the reason. You cannot know the perfect boiling temperature of your liquid, because you can't know the exact amount of different liquids in it (well.. technically, you could, but it would cost a LOT of money to find out). And even if you did, controlling the temperatures everywhere in your still down to the small part of a degree needed to make your distillation perfect, or even good, is also out of most of our reach. (Both of these are why commercial distilleries are very good at what they do. They can and do know to a very fine degree what's in their mash, and they have the equipment to control heat to a fine degree as well.)
Let me restate something from above: Once your pot is boiling, any added energy is being converted to vapor. This conversion from kinetic to potential energy means that the temperature (average
kinetic energy)
doesn't change. The only way to measure it is to measure the volume of the vapor being produced. And how do we do that? Yep.. by watching the speed of the condensate drip!
Yep.. simple, really, when it comes down to brass tacks. You may have thought it was magic, or maybe you're doing it by 'feel'. You tell the newbies, "Just watch the drip! It ain't rocket science!" Well, this time, it is!
