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On the other hand, temperature, volume and pressure are variables that are much easier in the lab to keep constant.
另一方面，温度，体积和压强,在实验室中比较容易保持恒定。
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And so an experiment said the gas didn't increase its temperature when it expanded the vacuum.
这个实验告诉你,气体在向真空膨胀的过程,中温度没有升高。
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Joule actually did this experiment, and he observed that for the gas expansions that he could do, that the temperature did not increase measurably.
事实上焦耳的确做了这个实验，他做到了,他能达到的最好实验要求，发现在可测量范围内没有观察到温度上升。
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Every time you do the experiment T in equilibrium with the heat bath at T, v2 you'll get the same p2 and V2.
与热库相接触的每次实验中,达到热平衡后的温度都是，压强都是p2，体积都是。
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Experimentally, though, that's not such an easy situation to arrange.
但是保持熵恒定在实验上很难实现，当然保持温度。
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It's tabulated in books, and this we can measure p in the experiment. Delta p here is the change in pressure from the left side to the right side, and we can put a thermometer, measure the temperature before the experiment and measure the temperature after the experiment.
这列在书上，这个量我们在,实验中也可以测量，在这里Δ,是从左边到右边的压强变化，我们可以放一个温度计，去测量实验前的温度,再去测量试验后的温度。
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We use a liquid helium. And so in order to make a liquid helium, you can't take helium at room temperature and do this, because if you did, you would just heat it up, because the room temperature is above the inversion temperature, so Joule-Thomson would heat up the helium.
为了得到液氦,不能再常温,做这个实验,否则就是加热氦气，因为室温高于它的转变温度,所以焦耳-汤姆孙，实验会加热氦气。
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This is real, unlike the Joule coefficient which is very small so that most gases have tiny Joule coefficients. So if you do a Joule experiment, you hardly measure a temperature change. With real gases, here you do actually measure it. You can feel it with your finger on your bicycle tire.
系数那样小以至于,大多数气体的焦耳系数,都很小，所以如果你做焦耳实验,很难测量出温度的变化,对于真实气体，你可以测量它，你能通过手指按在,自行车轮胎上来感觉到它。
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So you need first to take the liquid helium and cool it below 53 degrees Kelvin before you can do the Joule-Thomson to cool it even further to make liquid helium.
我们首先需要,把温度江都53k一下,然后利用焦耳-汤姆孙实验,来继续冷却它。
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Therefore, from experiments, u is only a function of temperature for an ideal gas, H and therefore from these experiments, 0 we come out with delta H dH/dp is equal to zero.
因此，从实验可以得出,对于理想气体u只是温度的态函数，因此从这些实验中我们得到Δ,偏H偏p等于。
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So in this experiment here, delta p is less than zero. You need to have this whole thing greater than zero. So delta T is less than zero as well. So if you're below the inversion temperature and you do the Joule-Thomson experiment, you're going to end up with something that's colder on this side than that side.
所以在这个实验中，Δp小于零,这全部都大于零,因此ΔT也小于零,所以如果在低于转变,温度的情况下做焦耳-汤姆孙实验,最后的结果是,这边的温度比这边低。
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