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