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3. Can some one tell me why Hot legs happen... I have been doing this a long time and never really knew the cause.
It it a little bit involved. A few rum and cokes so I feel like typing, lol. Frst, hot spots im general: The areas of the coil with higher resistance get hotter than areas of lower resistance. Any area on the coil shorted to the wick is generally lower resistance (as in shorted) and won't get hot. Hotspots are the areas of the coil not shorted to the wick. You can actually see this happen if you use the pulse method of building a genesis -- as the coil insulates itself from the wick by oxydizing it, you can see each loop that is no longer shorted start to glow. Except ... Heat transfer comes into play. If no areas of the coil are shorted, then areas of coil not efficiently transferring heat to the wick get hotter. If the coil is wound tightly on the wick, that's not the issue. Usually, the cold areas of the coil are shorted and the hot spots are not (unless its a really ugly not wound good coil).
Now on to hot legs. If the coil is shorted to the wick, the legs have higher resistance than the rest of the coil and will be the area to heat up. Once you have resolved shorts to the coil, if you still have hot legs them you have ... Heat transfer issues. Each loop of the coil transfers its heat laterally to the next loop. Hot wire has a higher resistance than cold so the hot areas of the coil will tend to get hotter. Once the distance between the loops exceed about 6 wire diameters, no area heats up a lot faster than the rest so the whole assembly, including the legs heats. Except ... (You guessed it, more thermal transfer) the legs dissipate a lot of their heat to the connection posts. If the legs are short, even though the distance between coil wraps is relatively large, the legs will dissipate heat into the posts and they won't get hot.
So ... To avoid hot legs: eliminate any coil short circuits to the wick, keep legs short, wrap coils so each loop is within at most 3 to 6 wire diameters apart -- i.e. keep the loops close to each other.
This is also why closely stacked coils (misnamed "microcoils") work the way they do. Each loop transfers its heat laterally and as it heats its resistance increases slightly -- enough to heat from center out and avoid hot legs. It also behaves more like a monolithic heater -- like one big heater even though its made of individual small loops.