. Applied thermodynamics for engineers. nal work wasthat due to doubling the volume of the air, or 1 x x 144 = The piston is now fixed rigidly in its original position,so that the volume cannot change, and no external work can be heat required to produce an elevation of temperature of °is then = B. t. u. The differenceof heat corresponding to the external work done is B. t. u.,whence the mechanical equivalent of heat is -j- = 73. Joules Experiment. One of the crucial experiments of the science
. Applied thermodynamics for engineers. nal work wasthat due to doubling the volume of the air, or 1 x x 144 = The piston is now fixed rigidly in its original position,so that the volume cannot change, and no external work can be heat required to produce an elevation of temperature of °is then = B. t. u. The differenceof heat corresponding to the external work done is B. t. u.,whence the mechanical equivalent of heat is -j- = 73. Joules Experiment. One of the crucial experiments of the science wasconducted by Joule about 1844, after having been previously attempted by Gay-Lussac. Two copper receivers, A and B, Fig. 9, were connected by a tubeand stopcock, and placed in a water bath. Air was compressed in A to a pressure of 22 atmospheres,while a vacuum was maintainedin B. When the stopcock wasopened, the pressure in each re-ceiver became 11 atmospheres, andthe temperature of the air and ofthe water bath remained practically. Fig. 9. Arts. 73, 80. — Joules Exi^eriment. unchanged. This was an instance of expansion without the perform-ance of external work ; for there was no resisting pressure against theaugmentation of volume of the air. 38 APPLIED THERMODYNAMICS 74. Joules and Kelvins Porous Plug Experiment. Minute observationsshowed that a slight change of temperature occurred in the water and Kelvin, in 1852, by their celebrated porous plug experiments,ascertained the exact amount of this change for various gases. In all ofthe permanent gases the change was very small; in some cases the tem-perature increased, while in others it decreased; and the inference is jus-tified that in a perfect gas there would be no change of temperature (). 75. Joules Law. The experiments led to the principle thatwhen a perfect gas expands without doing external work, and withoutreceiving or discharging heat, the temperature remains unchanged andno disgregation work
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