Not a homework question! I am doing exercises for upcoming final exam. So, I get stuck at question 5.27 (Griffith 4th edition textbook). Question: Find the vector potential above and below an infinite uniform surface current with constant current sheet, K flowing at positive x direction. I get really stuck so I googled the answer and it shows like this: http://www.physicspages.com/2013/03/11/magnetic-vector-potential-of-an-infinite-wire/ What I don't understand is, from the link there, why does equation (7) need negative sign? I mean, we already obtain a nice solution in equation (5). Help me please, thank you.