Why magnetic constant is specifically 4*pi*10^-7? I can understand other experimental values, for example it would make sense if it was 3.1415*10^-7. But 4*pi is something we can never confirm experimentally since pi has infinitely many digits. So there has to be theoretical reason for us to specify 4*pi. What is that? Only possible meaning I can think for 4*pi is the solid angle of sphere at the moment.