In December 1947, Eph Sparrow, then working on an MS, came to me asking if there wasn’t some small project he could do instead of going home for Christmas. Just before this time Dusenberie’s book on finite differences was published. There he solved a problem of conduction through insulation on a finite L/D cylinder. I suggested that we try to build a hydraulic analogy for the insulation. Eph made up five different diameter open-ended cans. With three setscrews near the top of the cans, one can could be placed inside the other on centers with different end gaps. Water added to the space between the cans provided different magnitudes of L/2. This provided a variety of magnitudes, Ri, R0, L/2, T0 = R0 − Ri.

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