  It is common practice to extract field enhancement factor from the slope of
FN plot. Many experimentalists working on field electron emission had reported
multi-(linear segment) FN plots, which can be divided into several (usually
two) linear segments. Then multi-(field enhancement factor) were extracted from
the FN plot. They claimed that the field enhancement factor increases with
applied field if the FN plot bends downward (vice versus if the FN plot bends
upward). We show that this is contrary to fact.
