transformed into work, or by which work is transformed into heat, is called thermodynamics.
67. An illustration of the conversion of mechanical energy into heat is given by the experiment shown in Fig. 10. A brass tube, about 7 inches in length and ¾ inch in diameter, is attached to a small wheel, and is rotated by means of a chord passing around this wheel and around a larger one turned by a handle, as shown; the tube is three- fourths full of water, and is closed with a cork. The tube is held by the clamp and made to rotate rapidly by means of the larger wheel; considerable friction is generated, which causes the water within the tube to be heated; the temperature rapidly increases, and part of the water is converted into steam, whose pressure becomes so great as to force out the cork.
68. Having shown that mechanical work can be changed into heat, it will now be demonstrated that heat can be changed into
mechanical work. Fig. 11
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represents a
cylinder AB partly filled with gas or air confined within the cylinder by means of the piston P. The gas is then under the pressure of the atmosphere, and has also an additional pressure due to the weight of the piston. If heat is applied to the bottom of the cylinder, the piston will gradually rise in proportion to the amount of heat supplied. In expanding, it will have to do work in order to raise the piston. Suppose that a rope, fastened to the piston and passed over a pulley, has a weight on the other end a trifle less than the total pressure of the atmosphere plus the weight of the piston. Now, if the gas within the cylinder is cooled, the piston will fall, owing to the combined weight of the piston and the pressure of the atmosphere, and raise the weight, thus performing work. In the first case, a certain amount of heat was supplied to the gas to do work; in the second case, heat
