I don't understand why this question was closed as lacking focus:

How can a thermometer ever show the actual temperature of an object if the object loses heat to the thermometer?

It seemed focused to me -- the OP was asking a single, straightforward question about thermometry: Doesn't the act of measuring temperature change the temperature? Of course, to many of us the answer is obvious. But what is obvious to us is not obvious to beginners. So here we have a beginner who is thinking about thermometry, and it occurs to him -- 'hey, wait a second, aren't I changing the temperature when I use a probe to measure the temperature?' To which the answer is yes, particularly when the heat capacity of the probe is significant relative to that of the object being measured.

I'll add that my answer was also downvoted, so apparently someone thinks I should have done a better job in that department; but I thought it was OK and, more importantly (at least in the OP's view), directly helped the OP's understanding (which was my primary goal).

  • $\begingroup$ Honestly, I wouldn't bother about downvotes, and there's only one on the question and one on the answer anyway. If there are 4 downvotes or something then there's probably an obvious reason, and I could make a guess, but I can't comment on the potential reasons for one single downvote. After all, re. the answer, you did get some feedback (whether justified or not, I don't want to be the judge). $\endgroup$ – orthocresol Jun 9 '20 at 4:42
  • $\begingroup$ For what it's worth, I don't agree with the closure, though. $\endgroup$ – orthocresol Jun 9 '20 at 4:48
  • $\begingroup$ @orthocresol Sorry, those were typos. I was wondering why the question was closed, not why it was downvoted. I've edited my question appropriately. $\endgroup$ – theorist Jun 9 '20 at 4:56
  • $\begingroup$ I agree with orthocresol about the closure; I can't even imagine how this question could be more focussed. $\endgroup$ – Martin - マーチン Jun 9 '20 at 9:16
  • $\begingroup$ It's been reopened, anyway. $\endgroup$ – orthocresol Jun 10 '20 at 12:27
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    $\begingroup$ Thank you, this is the OP. Your answer definitely helped my understanding, and I would like to apologize if the question was not focused enough. It was just a thought that I had reading a textbook on Thermodynamics, I'm sorry if it was too vague for this stack exchange. Thank you for taking the time to answer my question. $\endgroup$ – Neil Chaturvedi Jun 15 '20 at 15:48
  • $\begingroup$ @NeilChaturvedi Thanks for your post, it's the people who post questions as well as those that answer who make the site possible. Don't worry about this discussion, the mechanism for handling questions can be a bit vague and complicated and we are always looking to improve. $\endgroup$ – Buck Thorn Jun 17 '20 at 9:27
  • $\begingroup$ @NeilChaturvedi I hope we have answered your question. As one often hears said, "there are no bad questions" (even though we have guidelines on what questions should aim to be like and an editorial process that tries to enforce the rules). Yours certainly lead to considerable thought and discussion. I certainly learned something. The key takeaway for me is that science often requires attention to detail. $\endgroup$ – Buck Thorn Jun 17 '20 at 9:29
  • $\begingroup$ @BuckThorn I've learnt a valuable lesson too, and will improve in the future. $\endgroup$ – Neil Chaturvedi Jun 17 '20 at 15:54

I didn't vote to close this question but when I encountered it I lurked around for a while wondering if I should and then decided to draft an answer instead (now posted). What kept me from posting originally and put me off about the question (even though I have now posted an answer) is that it in fact still needs more focus but I suppose that can be regarded as a feature rather than a flaw. The author does not address experimental design and might even be unaware of the concept of thermodynamic equilibrium.

As I attempted to explain in the (now edited) answer to the OP: For the question to have a concrete answer you would have to explain in more detail how the temperature is to be measured. If you use a thermometer properly then it follows by the zeroth law of thermodynamics that the measurement recorded by the thermometer is the temperature of the sample. A thermometer, properly used, always shows the actual temperature of an object. That is the case when thermometer and sample are in thermal equilibrium. Many instruments and experiments aim to approximate this condition as closely as possible by using insulation and/or a large constant temperature heat reservoir.

  • $\begingroup$ Hi Buck. I think insisting "a thermometer, properly used, always shows the actual temperature of an object" is trivalizing what is a legitimate problem in thermometry, which is how to measure the temperature of an object in a way that minimally perturbs it. A measurement that shows the temperature after significant disturbance from the measurement is, yes, the current temperature, but is typically not what is scientifically meaningful. $\endgroup$ – theorist Jun 10 '20 at 1:00
  • $\begingroup$ Also, if you take the position that a true temp measurement can only be made at thermo equil, you restrict yourself to trivial cases: infinite heat baths and systems in thermal equil. with them. These are the only systems that are necessarily at thermodynamic equilibrium. In the latter case, temperature measurement is trivial, because their temperature is the same as that of the heat bath..... $\endgroup$ – theorist Jun 10 '20 at 1:07
  • $\begingroup$ ....The only other type of systems that can be (but might not be) at thermo equil. are thermally isolated systems. And you can't measure the temp of a thermally isolated system, since doing so would de-isolate it. Sure, you could put the thermometer inside the thermally isolated system, and allow it to re-equilibrate. But as soon as you try to communicate with it (to read it), technically you are no longer thermally isolated, and thus not technically at equilibrium. $\endgroup$ – theorist Jun 10 '20 at 1:10
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    $\begingroup$ @theorist I might be trivializing it somewhat. I realize there are examples where it is not simple to obtain an accurate measurement. But if you read the question carefully it leaves too many relevant details unclear. For instance, it starts by explaining the zeroth law (in layman's terms) and explains that the equilibration process involves a transfer of heat from a hot to a cold body. $\endgroup$ – Buck Thorn Jun 10 '20 at 1:10
  • $\begingroup$ However, if you've ever worked in a lab you've heard of heat baths and thermostats, they are essential for many instruments and experiments to provide reliable data. I grant that the question of equilibrium is bound to lead to argument, but the point is to clarify that if thermometer+sample are enclosed in a regulated T bath - a key feature of many experiments that require accurate T measures and control - then the thermometer is measuring the sample T. $\endgroup$ – Buck Thorn Jun 10 '20 at 1:15
  • $\begingroup$ More problematic are T gradients that are not necessarily due to heat being drawn away by the sample but rather by the impossibility of keeping thermometer and sample in a sample bath of uniform T. $\endgroup$ – Buck Thorn Jun 10 '20 at 1:16
  • $\begingroup$ Hopefully you understand now why I think this is about lack of detail (or focus, or whatever closure jargon is relevant). There are (relatively serious) logical flaws and there is a lack of information in the question. $\endgroup$ – Buck Thorn Jun 10 '20 at 1:18
  • $\begingroup$ Another serious problem is calibration of the thermometer, but that is another story. I would address these questions to the OP rather than me, BTW. As you could see I also posted an answer and did not vote to close because, as I mentioned, the flaws are a feature, an opening for discussion. $\endgroup$ – Buck Thorn Jun 10 '20 at 1:19
  • $\begingroup$ I should add that when I say thermometer I mean contact thermometer, as I suppose was meant by the OP (otherwise the original question would not make sense). $\endgroup$ – Buck Thorn Jun 14 '20 at 18:07
  • $\begingroup$ I recognize the ques. wasn't technically precise, and could have had more detail, but disagree it had "relatively serious logical flaws". I know you want to critcize it as logically flawed b/c of your position that a true temp. meas. can only be made at thermo. equil., but (1) if that were a flaw, it would be a technical one, not a logical one—a logical flaw would require internal inconsistency, and I don't see that in this post; and (2) if that's your basis for criticism, then that complaint can equally well be leveled at your fancier set-up, which is also not at thermo. equil. $\endgroup$ – theorist Jun 15 '20 at 1:40
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    $\begingroup$ Look again at the following statement: "If I put a thermometer in an object to measure its temperature, the body will lose its heat (or gain the heat, if the thermometer is hotter) until it reaches the same temperature as the thermometer. Doesn't this mean the thermometer is not giving us the EXACT temperature of the object, and is in fact, showing us a lower/higher temperature?" What is logically inconsistent is the idea that because the two bodies (at some point) exchanged heat, that you cannot (once that exchange is finished) determine the exact temperature. $\endgroup$ – Buck Thorn Jun 15 '20 at 7:37
  • $\begingroup$ @theorist There's nothing fancy about what I suggested. The principle is very basic. One comment captures the point in two words: "Thermal reservoir". As you pointed out it is also possible to perform non-contact measurements, but that does not address some misunderstandings on the part of the OP. $\endgroup$ – Buck Thorn Jun 15 '20 at 7:46
  • $\begingroup$ I now realize the reason you think his question was logically inconsistent is because you've misunderstood him. By "exact temperature" he means the temperature prior to measurement. That's clearly the whole point of his question. He is not referring to, or concered about, knowing the temperature "once that exchange [the heat exchange between the object and the thermometer] is finished". I.e., he's saying if something has a certain temperature, how can you ever know it, if the very act of measuring the temperature changes it? There's nothing logically inconsistent about that. $\endgroup$ – theorist Jun 15 '20 at 8:16
  • $\begingroup$ @theorist How can you know that? My point is that the question is unclear and my interpretation is afai can see the simplest. The OP refers to the zeroth law (equilibration) and to heat transfer. Let me give you another very simple example: if you measure your body temperature with a thermometer, will the temperature reading be affected by the heat capacity of the thermometer, or by equilibration? Is transfer of heat going to be a problem? Why or why not? $\endgroup$ – Buck Thorn Jun 15 '20 at 13:14
  • $\begingroup$ @theorist Please, I am finding this argument tiresome, I suggest you address additional questions to the OP rather than me, as I stated earlier. I have every right to vote to close (which I did not) if I see a question as unclear, and I've spent more than enough time trying to justify my view. $\endgroup$ – Buck Thorn Jun 15 '20 at 13:15

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