see title. I’m talking about mercury / alcohol thermometers off course, not digital ones. I assume it’s something to do with the inner shape of the tube, but why not a cylindrical tube that can be read from any angle?
Liquid expands or contracts slightly when temperature changes. If you put some liquid in a reservoir (bulb) connected to a really thin tube, a slight change in volume will cause the level to go up/down visibly.
For this to work, the tube needs to be very thin, which means it will be very difficult to see. So, to make it easier to read, the glass is shaped to act as a magnifying glass. But it only magnifies the tube when seen from a specific angle. If you get rid of the magnifier, it will be equally difficult to read from any angle.
From what I recall, you could see thru them from any angle (except the back, where the scale was), but only a small angle on the front magnified the image enough for it to be easily(?) readable. I suppose the glass is designed with a lens effect, to do this magnifying. I always found them pretty hard to read, especially as the temp would be going down as you held it out in the air.
OK, this explanation satisfied me at first (thank you), but then I thought a bit more about it…
The volume of a cylinder is pir^2h. If the volume changes, due to the temperature change, we’re looking for a change in h. pi is a constant, and the radius of the tube is constant along it’s length, so it seems to me that the height of the liquid in the tube should vary linearly with the volume at the same rate, regardless of how wide or thin the tube is.
What am I missing here?
What you are missing is that there is a bulb at the end containing the mercury/alcohol. It’s mainly that that expands and some of it is forced up the thin tube on the scale. The volume of liquid in the bulb is many times that of the liquid in the tube.
The tube of liquid is so thin that it is difficult to distinguish between the column of liquid and the empty space above it. And that’s what you need to find in order to know the temperature. But with this handy built-in magnifying glass, it will look a lot wider, and instead of searching for a silver or red line, you’ll easily see the silver or red rectangle.
(In this context, of course, “easy” presumes that you are holding the thermometer at the precisely correct angle to allow the magnifying glass to function. Often, I need to turn it back and forth several frustrating times until it hit it correctly. My guess is that when these thermometers began commercial production, there may have been a competition between different shapes. Looks like the magnifying glass won.)
OK… so? The volume of the bulb is fixed. Excess liquid that can’t fit in the bulb goes up the tube. So why does the tube need to be thin?
OK, I get the reason for the magnifying lens built in now, if the tube of liquid is thin. I’m still not getting WHY a thin tube is used instead of a thick one that would be visible without magnification.
Is it a cost issue? (because mercury probably isn’t cheap, although alcohol would be)
The tube is thin so that a relatively small change in the volume of the alcohol or mercury in the reservoir registers as a large change in the height of the column in the thermometer. If you used a large tube, you’d need a correspondingly larger reservoir of liquid. That reservoir would cost more, wouldn’t fit in as many things and would take longer to heat or cool so it wouldn’t register temperature changes as quickly. The magnifying glass seems like a better solution to the visibility problem.
The thinner the tube is, the more the height of the mercury in the tube changes per degree change in temperature. A wider tube would result in a thermometer with lower resolution (inches/degree) but a higher maximum temperature (takes a higher temperature to max out the thermometer).
You could use a wider tube and get the same resolution by increasing the volume of the bulb proportionally, but that would require more mercury which adds cost (and would be slower to respond to changes in temperature due to the increased thermal mass of the mercury in the bulb).
In addition, while I haven’t done the math, I strongly suspect that the volume of the tube must be much less than the volume of the bulb in order to get the most linear temperature reading.
Let’s say (because I’m not going to look up exact numbers here), that the volume of the bulb is 0.5mL and expansion of the fluid is 5% over a useful temperature range 0 - 100 degrees.
You have 0.025mL difference between extremes. This is a volume change and we need to make it a linear measure, and you do that by restricting the volume change to a cylindrical space.
Let’s leave pi out of it and calculate a couple of rectangular prisms instead. The first tube is 1mm x 1mm x 50mm long and contains a working volume of 0.05mL, so your visible difference between temperature extremes is 2.5cm total.
Let’s make the prism thinner, 0.5mm x 0.5mm x 200mm long, containing the same working volume. Again the change occupies 1/2 of the length which in this case is now a visible difference of 10cm. You now have four times the resolution.
This is why thinner is better.
-DF
For a given temperature increase, the bulb sends a specific amount (volume) of excess fluid into the tube. The thinner the tube, the higher that excess liquid goes.
Ahh… there it is. it’s not a fixed volume of liquid per degree that’s sent into the tube, it’s a percentage of the volume in the bulb!
So, it’s not any innate thinness of the tube that matters, but the ratio of the tube diameter to the bulb volume. Now it makes sense. You could make a tube wide enough to be read without magnification, but you’d need a really large bulb.
Thanks, now I get it!