The following is my understanding of hot legs, why they occur and why raking or cueing fixes them.
Alumina is a crystalline structure that forms on our resistance wire when heated that electrically isolates each turn from one another in a contact microcoil. When we tension wind a coil where the turns are held in a state of compressive strain (touching) some or all of the turns can be electrically shorted together untill the alumina forms during firing/pulsing. This shorting together of some or all the turns in a coil before the alumina forms completely results in hot legs and/or uneven firing of the turns. Why? Because the electrical current does not have to travel the entire length of the wire. Instead it takes the path of least resistance, jumping across from one shorted turn to another and bypasses a good deal of wire resistance. The small amount of heat generated in these shorted turns is readily absorbed by the greater mass of non-heated wire in the coil therefore no red color is developed in the coil but.....the legs will turn red because current is flowing through their entire length.
Alumina is quickly formed on the wire surface when the coil is heated but can only do so in the presence of oxygen. Both raking and cueing while the coil is hot are methods used to mechanically disturb the shorted junctions between the turns just enough to allow for oxygen to reach there. Alumina then forms in those previously unreachable areas and fully electrically isolates all the turns. The coil then fires from the center out and will likely never hot leg again.
Overly tensioned coils where the turns are held together with very high compressive strain are not required or desirable for the best vaping performance imo. Applying only enough tension to acquire diametrical uniformity and just enough wire feed angle to keep the turns in contact with one another seems to produce the best vapor production and coil longevity. I suspect that it has to do with the size and number of inter spacial voids created during the formation of alumina but alas.....I don't have access to micrography equipment to verify this.
Just to be clear, spaced coils never hot leg because there are no shorted turns. At least I have never seen it happen.
Hope this helps,
cig
