Talk:Thermoscope

image

When I look at the image, it doesn't seem to match the description or the photo, and I'm a little confused how the apparatus displayed could work given the physical properties. As I read the image, there's a closed sphere on top, with a fluid cylinder going into an open reservoir of fluid. If the movement is based on the expansion of the gases in the top sphere, then a higher temperature on the right (after all, T1 < T2) would push down the fluid, which is the opposite of the image. However, the air pressure is much more likely to have an effect than the temperature, right?

However, if I look at the photo, the reservoir is not open, but rather inside a larger sphere, closed. In that scenario, a temperature change would indeed shift the liquid column if the temperature at the top sphere is different from the bottom sphere (maybe even if it is the same? Not entirely sure there). Air pressure would at least not be a problem any longer.

Maybe I'm misunderstanding something here, please feel free to correct. (@CambridgeBayWeather, Eleassar, and Rpe19: ping, as first, frequent and image contributors) effeietsanders 07:02, 5 January 2018 (UTC)[reply]

The photograph shows a museum display in which the second entry to the bottom vessel is closed. This would be done in transit, but in normal use it would be open. The height of liquid column depends on the fact that the upper bulb has a lower pressure than that in the lower flask, which is approximately constant being at ambient atmospheric pressure. (The fact that the atmospheric pressure varies limits the accuracy of the device.) I surmise that the museum has closed the vent in order to limit evaporation of the liquid in a locked display cabinet. It is essential that the lower body of liquid communicates with the atmosphere. See, for example Home made air thermometer. Chemical Engineer (talk) 17:16, 5 January 2018 (UTC)[reply]

OK, that explains the open/close difference between the picture and the photo. I understand now that apparently the apparatus is specifically for outside temperature, and not the difference in temperature between the two spheres. However, I still can't wrap my head around how a higher temperature in the (closed) upper bulb would cause a lower pressure. After all, isn't p = nRT/V ? Anyone? effeietsanders 06:50, 6 January 2018 (UTC)[reply]

At least the sketch/schematic is not correct. Higher temperature leads to increased volume of the gas (pV = nRT), therefore, it should be T1 > T2. The T1 and T2 should be better placed outside of the liquid to avoid the misunderstanding that the temperature of the liquid is measured (in fact, the temperature of the surrounding air is measured). Pucicu (talk) 15:21, 16 August 2018 (UTC)[reply]

(Outdented for readability) I read the entire thread above, ending at 15:21, 16 August 2018, which seems to be by Pucicu, the creator of the line drawing schematic with T1 > T2. One major topic which has not been addressed: If the level of liquid in the tubes rises above the mouth of the tube (the very bottom), that can only happen one way: You need to fill the tube by pouring liquid into it while air can escape, almost definitely with the bulb at the bottom and the fluid going in the top. If you stick a tube mouth-down into liquid, the liquid will not rise into the tube because **none of the air can escape**. This step is never mentioned in the article, which seems like a major oversight. Many readers will see a picture like that and assume this works like a barometer (similar shape, but a barometer or manometer should have a near-vacuum above the liquid), but it's actually got a lot more forces and phenomena at work. Which brings up another topic:

The vapor pressure of the liquid has been ignored. Not only will air expand when heated, the amount of liquid vapor will increase with temperature, generally in a very nonlinear way.

If you lack a temperature standard, such as an ice bath (those stay near constant at the melting point of the ice due to the heat of fusion, as the ice melts, if you keep stirring, the ratio of ice to water changes but the temperature stays fixed at just about the freezing point of water), if you lack a temperature standard, it would be very hard to set up two separate devices as pictured that matched. This doesn't control for the temperature of the liquid as you pour it into the tube-bulb, nor the starting temperature or humidity of the air.

This device would also be wrecked if you ever turned it upside down. I'm not sure anything like this was ever used to measure anything, but they may have been toys or displays similar to lava lamps. I could believe that. I don't have my password on my phone sorry. 209.94.144.13 (talk) 15:40, 4 November 2020 (UTC)[reply]