Min/max thermonitor

[Si]

I've a min/max themonitor.  It works fine: there are two columns: one with a normal scale and one with an inverted one.  each column pushes up a little "float" and the float stops at the highest and lowest temps respectively until it's all reset.

So far so good.

But it's starting to really annoy me as I can't work out how the min column works.  OK the max is just a bulb of mercury that expands in the heat and rises up the scale.  But the min one looks to do exactly the same - i.e. rises as the temp goes down...that's not right is it?

Go on, please, someone explain to me how the min gauge works?

thanks

[Wowbagger]

On one side you have a bulb filled with some sort of alcohol (I think).

Then you have a band of mercury.

Then some more alcohol but the bulb at the other end is a bubble of air. This bubble becomes compressed at high temperatures as the alcohol at the other side expands.

On each side of the mercury is a glass rod with some very fine iron wire within it. A magnet behind the graduations holds the glass rods where the mercury leaves them, thus recording the maximum and minimum temperature. When you press the button to reset the thermometer, the magnet releases the glass rods which gravity then forces back to the level of the mercury.

There are other designs in which the glass rods are held within the tube by tiny hair-like springs, and when you reset the thermometer you use a magnet to pull the rods down again. The disadvantage of this design, as mu father found, is that your naughty little grand-daughter can take this magnet and drop it down the well.

[Si]

thanks, I think that I follow!

[Greenbank]

Thermometer