Question: The magnetic circuit shown (in the pictures) is constructed with a semi-circular piece of metal and a metal bar. The c.s.a of the metal parts is 10mmx10mm. The coil has 500 turns and a resistance of 10Ω. μr of metal parts = 1500. μo = 4π x 10-7 My attempt: I completely understand the concept of how to work out the flux, but there's just one part that I can't seem to understand. 1. The first picture shows the diagram. 2. The second picture shows all the values I obtained throughout the entire solution. 3. The third picture shows the working out of the reluctance of the semi circle. 4. The fourth picture shows the working out of the reluctance of the other piece of metal and the 2 air gaps. 5. The last picture shows me adding the reluctance of all the parts together and working out the answer, which is the flux. The working in the third picture is what I don't understand. I got this from a friend and I don't understand how he works out the "mean radius" and "mean circumference." Where does the 3cm - 0.5cm come from? Can someone please explain?