Re: Mercury thermometer stuck at full scale. Help please

Dan, I think the news is not good. I would be quite confident that your diagnosis of a 
break in the continuity of the liquid at the neck of the bulb, between the bulb and the 
thread, is about right.

But there are a few things that need to be clarified.

(1) Are you sure it is a mercury thermometer? You describe the liquid as a "blue line".
In the 1950s and 1960s, many thermometers used an oil dyed red or blue instead of
mercury. It was both cheaper and less toxic, and it did the same job. A real column
of mercury looks silvery and metallic, and definitely not blue.

Neither mercury nor the oil that was used as a substitute adheres at all well to glass.
Mercury coheres very well to itself, and a mercury thread is unlikely to break without 
extreme provocation. The oil, on the other hand, is not particularly cohesive, and it 
may well slowly oxidize in atmospheric conditions to other substances that do 
adhere to glass.

There are two types of vintage/antique barometer that you may be referring to. If you
have an Aneroid barometer -- 10-20 cm, round, rotating needle scale -- the 
thermometer is almost certainly an oil thermometer, and it cannot be fixed. The good
news is that oil thermometers like this are cheap, and it probably would not be hard 
to find an appropriate replacement. At the very least, you will need fresh oil anyway.

It is also possible that you have a Torricelli/mercury/Fortin barometer. Such an
instrument would be about 1 metre tall and 1-2 kg weight, probably only about 5 cm
wide. It would have a visible mercury column, whose height you would read off a ruler 
scale, sighting onto an ivory sightboard. And it would probably have a small mercury 
thermometer mounted at the top, beside the sightboard. In this case you will have a 
genuine mercury thermometer.

(2) The things you have tried are more or less the right ones. The freezer will not help.
It is NEVER a good idea to heat mercury above room temperature except in a specially 
equipped lab. Mercury vapour is extremely toxic by inhalation (if you want to follow
this up, some research on the "Mad Hatter" would be appropriate). Having said that,
I will also say that heating the mercury to about the top of the scale (no further), and 
tapping it around a bit while it is hot, is probably the best chance of rejoining the 
thread. I am a little surprised that it didn't work. The fact that it did not suggests to
me either that your thermometer does not really contain mercury, or that there is a 
minute crack in the bulb, and some air has got in. A vibrator is unlikely to work, but 
it might. Ditto for an ultrasonic probe -- better chance than a vibrator, but still not 
much.

I hope this helps, and that you can manage to find some way to resolve your problem.

John.