A system for heating $0\mathrm{.}1$kgs$-1(1\mathrm{.}01\backslash$times $10-5$m$3$s$-1)$ of water from an inlet temperature Tm$,$i $=300$K to an outlet temperature of Tm$,$o $=330$K involves passing the water through a thick$-$walled square tube having inner and outer diameters of $20$ and $40$ mm respectively and a length of $20$m$.$ The outer surface of the tube is well insulated, and electrical heating within the wall provides for a uniform heat generation rate