The bar is clamped in a metal vise, so the vise is very slightly heated and that side of the bar is much cooler.
Taken using the Topdon Android attachment.
i genuinely think schools should teach you that metal can get hot like this, it never occured to me until i was using a drill to screw in a screw and it slipped a bunch, then i went to touch the screw..
nice + shaped blister right on the middle of my fingertip :)