Hi. I would like to ask a simple question. Here is the link of the file I study on. Immediately before the formula 4.9 for Biot number. Lc=V/As but I cannot understand it and I think it is not clear enough. How it appears, for what the word "characteristic" stands for, for example a pipe? For what V and As stands for? Is that characteristic length for only "spherical objects" or it is valid for cylindrical objects? File: http://kntu.ac.ir/DorsaPax/userfiles/file/Mechanical/OstadFile/Sayyalat/Bazargan/cen58933_ch04.pdf Thanks.