From: dhky@shaw.ca   
      
   Tomtech wrote:   
    > If you  think using an iron core with infinite permeability would   
    > reduce the "leakage" flux, then we are at an impass, until it is   
    > cleared up.   
      
   It would have little effect as the leakage is dominated by the external   
   flux path. Reduction of leakage would depend on the spacing between   
   primary and secondary, and the spacing between these and the core, as   
   well as  the distribution of the windings around the core- your   
   interleaved windings on a toroid are an approach to that. Somehow, the   
   fact that there is a need for insulation is also a factor.   
      
   I have had some thoughts about this and I missed the most important   
   factor which makes my answer wrong.   
      
   First of all -using Faraday there is a direct relationship between the   
   applied voltage of the primary and the total flux (phi)enclosed.This   
   flux can be all in the core, partly in the core or outside the core.   
   This is independent of the core. However, relative permeability and the   
   flux paths determine the distribution. Now suppose that the core   
   permeability approaches infinity. The magnetic flux doesn't change as it   
   is tied to the applied voltage.   
   Now B=muH  or H=B/mu and this implies that the H approaches 0 in the   
   core. Total NI magnetizing also approaches 0 and that means that the   
   integral of H.dl along any and all flux paths linking this winding so   
   the for a path that is not fully in the core the flux goes to 0. That   
   is- no leakage flux.   
   Loading the secondary doesn't change this situation.   
      
   --   
   Don Kelly   
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