The central part of the concave heating surface defines {HEAD ~ the lowest point} of {TAIL ~ the heating surface} and the peripheral part of the concave heating surface defines the highest point of the heating surface 
of
The central part of the concave heating surface defines { HEAD ~ the lowest point } of { TAIL ~ the heating surface } and the peripheral part of the concave heating surface defines the highest point of the heating surface
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH