Jim wrote "My Max/min thermometer does not give equal readings on
the two scales.Any idea how I can equalise them please? It is a mercury
thermometer with markers in each section of a U-shaped tube.
These can be reset by an inbuilt magnet at the back. .........."

To put it simply....you cant equalise them.
BUT.........if you look carefully at the glass U you may see 2 marks cut
onto each side of the tubes, one at 32 F and the other at around 80 F (well
you always did in the "Old days") .
I always use the Min temp for recording the low temp. as long as it is below
60F and the Max for highest as long as it is over 60 F.
With my thermometer the difference is just one degree F so it is easy to
compensate.
Older thermometers were always checked when they were made and the 2
settings marked on the glass tube.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk