Page:Astounding Science Fiction (1950-01).djvu/110

 experimental evidence to show that this does not occur. There have been quite a few theories of the operation of the "Vortex tube" proposed. The explanation which the author prefers is that of Dr. John R. Roebuck, Physics Department, The University of Wisconsin.

Dr. Roebuck points out that due to the manner of entry, the air in the top of the T-tube is rotating very rapidly about the axis of the tube. This rotation sets up large centrifugal forces on the air particles outward from the axis, which produce a drop in pressure toward the axis, and a lower pressure at the axis than at the periphery. This is confirmed experimentally, as air will enter at the axial opening instead of escape unless the opening at the bottom of the whirling tube is constricted somewhat. The air coming in at the periphery forces the air in along the radii, and the falling pressure inward on the radius allows this air to expand doing work and cooling the air. It is readily shown that this work adds to the rotating energy of the body of the gas, so speeding it up. Friction with the inner surface of the tube heats up the gas next it. That is, gas drawn off at the axis has been cooled while that drawn off at the periphery near the outer wall has been warmed. Adiabatic expansion of air from ten atmospheres and room temperature to one atmosphere will cool the air by about 135°C., so there is plenty of available cooling.

And thus the charming picture of the busy little demon sitting inside the tube with a tennis racquet in his hand, batting the molecules to right and to left as they come in, saying to himself, "There's a hot one, smack it to the right! There's a cold one, smack it to the left!" is very effectively demolished, and the Second Law continues to hold true without known exception.



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