I was looking at a fire alarm at my school the other day, and I happened to notice that there is a small bar that breaks when you pull it. That bar appeared to be filled with some sort of red ink. How come that ink is there? I figured it might’ve been to try to “tag” whoever pulled the arm, thus coming in handy if it were a prankster, however, the way it’s designed you would actually have to work to get the ink on yourself…Any ideas?
A Freudian slip is when you say one thing and mean your mother.
My WAG: it’s mercury. The little bar filled with “red ink” you describe sounds an awful lot like a mercury fever thermometer. Mercury is a great conductor of electricity and is used (or at least used to be) in light switches. Electricity flows in a circuit
from some sensing or alarm equipment through
the mercury in the bar. When you pull the alarm, the circuit is broken and the equipment senses this, sounds the bell, and
alerts the fire department.
Why use this method rather than a switch, which is reusable? Well, for starters, a fire alarm is something that you want to stay on, once turned on. Additionally, my guess is that actually breaking the bar, which needs to be replaced at some expense, is a greater psychological deterrent to the pranksters you mention than a switch which they can flip on and off at will. Incidentally, you break the bar, turning the sensing circuit OFF rather than directly turning the alarm ON, because the alarm is designed to go off if fire or some other catastrophe burns or breaks the wires, a good thing if no one’s in the building or no one notices the fire.
Why use mercury, which is poisonous and definitely not fun to clean up? Maybe
you’ve got an old alarm. The ones in my apartment building have a thin wire running through the bar, similar to wire mesh security glass.
In my home sprinkler system, there are glass vials filled with alcohol which keep the valve at each sprinkler head closed.
The purpose of these is that a fire will heat the alcohol, causing it to boil/expand, which will break the vial, which will then fall out and allow the valve to open. Fortunately, I haven’t had to try this out myself. I saw a similar system demonstrated on an episode of “This Old House”.
Perhaps your school fire alarm works on a similar principle, so that it will trip by itself in case where no one’s around.
Nothing to add except that perhaps the fluid is a red electrolyte so that it can be checked for leakage. I don’t think that the red mercury in thermometers is mercury however.
Oh and a big welcome to the SDMB to all of you.
A point in every direction is like no point at all
Mercury is silver. The red stuff in thermometers is water dyed red. (Because it’s in such a small, tight space, it expands and contracts with a temperture change. The red stuff in your fire alarm is most likely something to ‘tag’ whoever pulls it. When I was in high school a rash of false alarms led the administration to put an indelible blue dye with the consistancy of petroleum jelly on all the fire alarms. That was real smart. It only took about five minutes for anyone to realize that a paper clip fashioned into a hook would pull the alarm without tagging the fiend. That and kids were scraping the stuff off the alarms and throwing it at each other. Nasty stuff it was, too…
The colored dye may also be used to mark the location from whence the alarm was initiated… There are some pull stations that can be almost pushed back up to look like they aren’t the ones that have been pulled. (I have a decent amount of experience looking for alarms that are pulled during blackouts here at school). And, if you’re in a hurry to shut the alarm off, it can be easy to miss if it’s been pushed back up! The dye would certainly be easier to spot!
“Far better is it to dare mighty things, to win glorious triumphs, even checkered by defeat, than to rank with those poor souls who neither suffer much nor enjoy much because they live in the gray twilight that knows neither victory nor defeat.” Theodore Roosevelt