\footnoterule
0.4pt
thick (or wide), and
0.4\columnwidth
long in the standard document classes (except
slides, where it does not appear).
\footnotesep
\footnotesize
fonts (see Font sizes), therefore there is no extra space between
footnotes. This is ‘6.65pt’ for ‘10pt’, ‘7.7pt’ for
‘11pt’, and ‘8.4pt’ for ‘12pt’.