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The ClueGerman physicist of 1800's (7 letter answer)
NotesThe answer FECHNER is so rare that it appears less than once every 2500 puzzles.
DefinitionFECHNER as a noun:
1. (Fechner, Gustav Theodor Fechner) = German physicist who founded psychophysics; derived Fechner's law on the basis of early work by E. H. Weber (1801-1887)