That leads us to the next point regarding this principle of transformation, which is that all transformations of energy take place in fixed proportions. When a certain quantity of coal is burned, a certain quantity of heat, or thermal energy as it is sometimes called, is produced, and the quantity of heat so produced is definitely proportionate to the quantity of coal consumed. If a certain quantity of coal were burned in a perfect steam-engine, that is one in which there would be no loss of heat, then also a definite amount of mechanical work would be done, which would be strictly proportionate to the heat generated by the consumption of the coal. So that when coal is put into an engine, the potential energy of the coal is transformed into kinetic energy of the steam, and that is again transformed into actual mechanical energy of the engine itself, by which work is done in driving or pushing or pulling the train along, and the amount of work done is proportionate to the coal consumed. Illustrations of transformation are common, and may be seen by any person living in a large town. Thus at any electrical station or electric tram terminus, these transformations of various forms of energy are very familiar sights. We have first the transformation of the coal in the furnace into heat. This heat converts water into steam, whose motion is communicated by proper machinery into a dynamo, the product of which is electricity. That electricity is then conveyed along wires, and work is done by it, by moving trams along the connected tram system, or it may be converted into heat in the carbon filament in the car itself, which, if heated enough, will then produce the electric light. So that starting from the coal, we have several transformations therefrom into the forms of heat, light, motion, and finally mechanical energy, which results in Work. The question arises as to what is the law of equivalence in regard to the transformation of energy. That is, if we have a certain amount of energy of a given sort, how much of any other sort can be produced by it? The answer is partly to be found in a statement made by Joule in 1843, which practically embodies what is known as the first law of Thermo-dynamics, and is as follows: "When equal quantities of mechanical effects are produced by any means whatever, from purely thermal sources, or lost in purely thermal effects, then equal quantities of heat are put out of existence or are generated, and for every unit of heat measured by raising a pound of water one degree F. in temperature, you have to expend 772 foot-pounds of work." From this law we learn that heat may be used to do work, but that a certain amount of heat is always used up in the process. It can also be demonstrated that electric currents can do work, but to generate the currents a certain amount of work must be done.