I DOWNLOADED a BBC World Service programme on the Mathematics of Fever this January, made notes and did more reading.

Then I quizzed last Monday’s dog walking group: did they have the means to record their temperature at home should they become ill?

I planned to write this column the following day.

On Monday evening out came my trusted mercury thermometer which belonged to Father and which I take everywhere with me (even to France in the base of my bicycle’s handlebar bag).

It recorded 100’F / 37.7’C… I’m now recovering from flu.

Three days of temperatures as high as 102’F/38.9C – traditionally anything above 100.4’F/38’C is considered a fever.

In today’s photo you can see this thermometer - and if you are over the age of 42 and received medical attention from Father I suppose it’s even possible it was once used on you.

These days I would hesitate to put a thin piece of glass containing such a poisonous metal as mercury into my grandchildren’s mouths – yet for over a century, lest we forget, this was the norm.

It wasn’t until the 19th century that Carl Wunderlich went against his mama’s wishes and became a doctor (she wanted him to become a priest).

Awarded the Chair in Medicine at Leipzig, he developed a clinical thermometer calibrated to a base-line norm of 98.6’F.

Wunderlich collected a million plus measurements over 18 years on more than 25,000 patients and his data set was so large it was never questioned until very recently.

He recorded axillary (armpit) temperature rather than sub-lingual; and an axillary recording is lower than if one were to take a sub-lingual one at the same moment.

I was delighted to hear this on the programme: the ‘new norm’ is now set at between 97-99’F /36.1- 37.2’C. ’Twas music to my ears because it vindicated my own observations: namely that my sub-lingual ‘norm’ on this trusty old steed of a mercury thermometer is about 96’F and that I feel unwell before the arrow is reached.

One of our two daughters is the same, the others in the family don’t appear to be unwell until their temperatures are a little higher, and one son operates effectively even with a very high fever.

There is a degree (excuse the pun) of variability from person to person which is now recognised. It’s good to understand your own ‘norm’…

Back to those dog walking friends. A third of them have no means of recording their temperature.

To know with a degree of accuracy whether your fever is increasing (not good) or whether you are holding your own (better but still debilitating) is a useful tool should you need to contact a doctor.

A cheap and cheerful clinical mercury-free thermometer starts at about £4, and digital ones will cost you more.

Personally I like ones without batteries – unless you are vigilant the one time you want to record your temperature may be very the time the batteries need changing…

No household should be without one.