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Brilliant! Thank you!! I've changed everything and it works and is a lot faster now!Manipulate[ ContourPlot[   Magfield[{x, y}, a], {x, 0, 3*10^-9}, {y, 0, 3*10^-9}], {a,   5*10^-10, 15*10^-10}]One quick question, does it make sense...
Thanks! Seems I should have put: A[{x_, y_}] :=