Searle's experiment for measuring the thermal conductivity of copper

[BOAS]

Hello,

i'm doing my prep for my lab experiment tomorrow and have a few questions about the experiment.

The schematic is attached.

My lab script says the following;

Once the steam is flowing well and thermometer T₁ has started to rise significantly, adjust the rate of water flow through the heat exchanger until it flows out of the outlet at a few hundred cm³ in 3-5 mins (the precise rate does not matter). The flow rate should be such that the constant level cistern stays filled up to the level of the overflow pipe, but without a huge amount overflowing.

2 questions.

It says the precise rate of flow of water through the heat exchanger is unnecessary to know. Is this because I will be collecting all the water so will know it's mass?

What is the purpose of the constant level cistern? I think this might become clearer when I actually see the apparatus but i'd sleep easier tonight knowing that I understand it's purpose.

Thanks for any help you can give!

 

[Philip Wood]

(1) Yes, but also the instruction is telling you that the experiment will work just as well at higher or lower flow rates (within quite a wide range).

(2) Although the exact flow rate doesn't matter, you do need it to stay the same once you've set it. If you simply connected the heat exchange pipe to the water tap the flow rate might not stay the same; e.g.. it might be affected by someone else in the lab turning a tap on or off. The constant flow device keeps the flow rate constant because it gives a constant pressure difference between the two ends of the heat exchanger pipe.

The apparatus in the diagram is exactly the same as the apparatus I used at a school a very long time ago, and it probably hasn't changed since G F C Searle devised it, perhaps a hundred years ago [1905, I believe]. A design classic!

 

[BOAS]

That makes a lot of sense, thanks!