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A Leslie’s cylinder for a couple of quid.

PostPosted: Tue Jun 17, 2014 8:27 am
by JohnG7OHO
You don’t need to spend $400 for Pasco’s Leslie’s cube ( ... lies-cube/) or £40 from Philip Harris. An empty baked bean tin will do just the job! Make sure the lid is still attached though. I painted one half matt black from a tester pot, for £1.99. Add boiling water, stir occasionally. I used a thermocouple and obtained these readings: shiny side 0.2 mV, black 1.3 mV. A dramatic difference.
You don’t need a thermocouple to prove that a black surface is a better emitter of heat radiation than a shiny surface - just bring the palms of your hand close to the can. The important point is that both sides, shiny and black, are at the same temperature. The water needs to be stirred, the wall of the can is a good conductor - so both halves of the can are at the same temperature. That’s the whole point of the Leslie’s cube. The heat radiated from each surface is different.
We mustn’t trust our senses to guess a temperature (remember the three bowls of water experiment!) but if you momentarily touch the outside of the can the shiny side actually feels hotter than the black! You mustn’t trust an IR thermometer either! Pointing it at the two surfaces gave these results: shiny 31oC, black 89oC. I don’t think so!