ENERGETICS. are commonly implied in the principle of con- Ben ation of energy. Historical Sketch. Although the concept "i force as the effort made in doing work, or that of energy as the capability of a body to do work, might either have been made the starting-point lor a system of dynamics, the former, which was the Newtonian, was first and most completely developed. This seems to have been owing to the fact that force appeals to our so-called 'muscular sense' or sense of muscular effort, whereas there is no distinct sense-perception of work or energy. When brandies of physical science other than mechanics were found to be related to mechanical work with a defmiteness that had not before been suspected, the Held of energy was widened and the Huygenian concep- tion became a more familiar one. The first great step of this kind was the recognition of an identity equivalence of mechanical work and of heat-effects produced by such work. Experiments by Count Rumford in 1798, on heat produced by the boring of cannon, and by Sir Humphry Davy, in 1799, on melting of ice by friction, introduced the idea that heat is a form of energy for which there is an exact mechanical equivalent. This proposition, con- troverting the then accepted theory that heat is material, was too radical to meet with wide acceptance, and the subject received little further development for nearly half a century. It was reasserted by Julius Robert Mayer in a philosophical discussion in May, 1842, but his determination of a definite numerical value for the mechanical equivalent of heat was not made public until 1845. To measure the unit chosen was the 'quantity of heat' necessary to raise a unit mass of water one degree in temperature Taking for the unit mass one gram, and the Centigrade scale for temperatures, the heat unit is called a calorie (or sometimes a watergram degree). The unit of work in gravitation meas- ure may be taken as the work of lifting one gram weight a height of one meter, called a gram-meter. In the various experiments other units were employed, but we give the results, re- duced to these. The problem was to determine how many gram-meters ran produce one calorie. ami are therefore equivalent to it in energy. This number is called the mechanical, or better. the dynamical equivalent of heat. It may of course he expressed finally in absolute units of work. Mayer's value, 365, was obtained by ob- serving the heat evolved in compressing air. In January, 1843, James Preseott Joule read a paper before the Philosophical Society of Man- chester regarding the thermal and chemical ef- fects of an electric current, which was followed by other investigations in rapid succession, and on August 21. 1S40, he communicated to the Brit- ish Association for the Advancement of Science the result of a most significant investigation, in a paper "On the Calorific Effects of Magneto-Elec- tricity, and on the Mechanical Value of Heat." He obtained for the latter 460 gram-meters. Then, by allowing the work to be done by weights descending under gravity, and the heat to be pro- dueed by the friction of water forced through narrow tubes, he obtained the value 423 gram- meters. In November of the same year a Danish engineer, A. Colding. presented before the Acad- emy of Copenhagen the results of experiments upon the heat produced by friction of solid bod- 71 ENERGETICS. ies, and expressed the view that the law of con- servation of force was a general one. His result for the 1). E. was 370. Although the principle of conservation of energy was suggested almost simultaneously by several physicists, Joule was most indefatigable in the prosecution of his exper iments, and within two years he had determined the dynamical equivalent of heat by a variety of methods, the most celebrated of which was the employment of descending weights to drive pad- dles in a vessel oi water, the latter being heated by the friction of the currents in the water pro- duced by the vanes. His paper giving an account of this determination was published in the Philo- sophical Magazine in 1845, and has become elassie. In 1847 appeared a discussion of the subject by Helmholtz, entitled "Ueber die Erhal- tung der Kraft," which contributed greatly to the establishing of the principle. Between that time and I860 the whole subject was discussed theo- retically and extended experimentally by Helm- holtz, Joule, Rankine, Thomson (Lord Kelvin), Clausius, Maxwell, and many others. By 1850 Joule had obtained the value 423.55 gram-meters as his best result, and that number stood as the most acceptable value for more than twenty years. In this interval, however, many experi- ments were made to determine this important quantity by transformations of energy through mechanical, electric, magnetic, and chemical pro- cesses, and by 1860 the generally accordant re- sults had conclusively demonstrated not only that heat is a form of energy, but they also demonstrated «the conservation of energy. The effect upon scientific investigation was extraor- dinary. "From now on, one was in possession of a principle which, tested in all known realms by careful experiments, offered now an excellent guide also to wholly unknown and unexplored regions." (Planck.) "It is the one generalized statement which is found to be consistent with fact, not in one physical science only, but in all. When once apprehended, it furnishes to the physical inquirer a principle on which he may hang every known law relating to physical ac- tions, .and by which he may be put in the way to discover the relations of such actions in new branches of science." (Maxwell.) So important a constant is the dynamical equivalent of heat that attempts to determine its value have been made in many ways, including among them various indirect methods in which heat is pro- duced electrically or otherwise than by mechani- cal work directly; but the most elaborate re- determination by Joule's method of the friction of water by stirring was made in the years 1877 to 1879. by Professor H. A. Rowland, in Balti- more, Md., U. S. A. His results are, on the whole, the most acceptable, and give not only a highly accurate value, but bring out the differ- ences in the value owing to differences in the specific heat of water at different temperatures. They range from 429.8 at 5° C. to 425.8 at 36°, passing through a minimum value of 425.5 at 29°. To express these values in ergs they must be multiplied by 100 times the weight of one gram in dynes at the place to which the results apply. This weight is numerically equal to the acceleration of gravity in ems. per see-. t Baltimore, g = 980.05 and Rowland's mean value of the dynamical equivalent from 20° C. to 35" C. is 425.9 gram-meters, or 4.17 X 10' ergs. The
