OK, merci David.
Le lien donné pour l'explication de l' "entrefer distribué".
http://www.diyaudio.com/forums/showthread/t-39745.html
"There is another way to get an effective magnetic gap by co-winding the M6 material with a mylar strip film. This is usually done for mass produced cores since the "gap" cannot be adjusted afterwards, but is more mechically stable over time than cut gaps, and simplifies matters for automatic winding equipment. (Since it is a bad idea to wind near any cut gap, very high copper losses near any gap - eddy currents induced in the copper , to the point of overheating and insulation failure in a power core.) But your winder may be able to find a catalog distributed gap core with a reasonable characteristic."
"The magnetic flux does have to actually cross the interface between M6 layers to make a loop, it doesn't just spiral out along the sheet. Normal cores are tightly wound M6 strip for this reason, usually spot welded at the outer end to hold them tight or baked with some epoxy binder. So if you should ever want to remove some core material on a DIY project, remove M6 from the inside of the toroid if it has a spot weld on the outside, so it stays tight. The reason the usual toroids act gapless is because of the large area available to cross layers and tight fit. Sort of like Ohms law for resistors; the gap width, or 1/(tightness of wind) in this case, is series resistance, the larger gap area is like paralleling resistors."
