The table below gives the equilibrium temperatures we obtained with various disk covers, water layers, and lamp covers. For more details, including annotated graphs of temperature versus time, see here.
Here are two highlights from the results. First, a 100-μm film of water on the top surface of our brass disk, contained beneath a cover of kitchen cling-film so that it cannot evaporate, causes the disk to cool by 1.7°C. The water film is a far more efficient radiator than shiny brass. Second, we observe negative feedback during the radiative cooling. Most of the heat from the lamp leaves the disk by convection through the top surface, so as the water film cools the top surface by radiation, the convection slows down, and heat is held back inside the disk. As a result, we see a gradual decline in temperature instead of a first-order exponential step. (You can see this decline after Point 10 in this graph.) The negative feedback, and the fact that a thicker film of water acts as a thermal insulator, make our observation of radiative cooling at the top surface hard to perform. With cling-film and a syringe, we can now reproduce the effect at will. But a better choice would be to coat the bottom surface of the brass with water, and that's what we will do next.
Nevertheless, what we have shown is that a water film cools down an object that is being heated by visible and near infrared light, but which is itself a poor radiator of far infrared. We have also shown that simple systems like our brass disk and lamp can exhibit negative feedback.
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