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For Joule's work it was, done in the fifth decade of the century, which demonstrated beyond all cavil that there is a precise and absolute equivalence between mechanical work and heat; that whatever the form of manifestation of molar motion, it can generate a definite and measurable amount of heat, and no more.
During this time, it is true, other molecules emerge from the space enclosed by the circuit; but the two effects do not counterbalance each other, and the resulting current is maintained. There is elevation of temperature in the circuit in accordance with Joule's law; and this phenomenon, under such conditions, is incompatible with the principle of Carnot.
This equation, called "Joule's equivalent," or 1 thermal unit = 772 foot-pounds, is the foundation and the corner-stone of thermodynamics. It is essential to understand the meaning of this equation.
By Joule's great discovery that the same amount of work, whether mechanical or electrical, and however expended, always produced exactly the same amount of heat that, in effect, heat and work were equivalent and interchangeable the way was opened to the conclusion that the total energy of the material universe is constant in amount through all its changes.
When a cannon-ball strikes the side of an iron-plated ship, a flash of light shows that collision has converted the motion of the ball into intense heat, or when we jump from the table to the floor, the temperature of the body is slightly raised, the degree of heat produced in both cases being ascertainable by the application of Joule's law.
Multiplying this by 425, or Joule's equivalent for the metrical system, the energy developed in heat is given by Dividing T1 by T, we obtain the ratio which the energy developed in heat bears to the total energy of the blow. With regard to the form of the zone of melting, it was found always to extend round the edges of the indent produced in the bar by the blow.
Thought has, in many directions, been profoundly modified by Mayer's and Joule's discovery, in 1842, of the equivalence between heat and motion. Its corollary was the grand idea of the "conservation of energy," now one of the cardinal principles of science. This means that, under the ordinary circumstances of observation, the old maxim ex nihilo nihil fit applies to force as well as to matter.
Joule's first experiments clearly proved that each of these forms of energy was convertible into the other; but some discrepancies arose in determining the exact equivalent of each. His subsequent researches, however, clearly demonstrated the true relation between both.
Joule's experimental proof of the elongation of iron by one seven-hundred and-twenty-thousandth of its length when magnetized, proves at least that its form is not spherical; and, as I am unable at present to demonstrate my own views as to its exact form, I have simply indicated its polar direction by arrows the dotted oval lines merely indicating its limits of free elastic rotation.
Now Joule's and my own old experiments on the efflux of air prove that if the crowd be common air, or oxygen, or nitrogen, or carbonic acid, the temperature is a little higher in the denser than in the rarer condition when the energies are the same.
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