MECHANICAL EQUIVALENT OF HEAT
64. Conversion of Heat. — Heat may be described as the lowest form of energy, into which all other forms of energy tend to descend. In almost every conversion of energy from one form to another, there is a certain amount of waste, and this waste usually appears as heat. For example, the friction of any machine appears as heat, but at a temperature too low for it to be used, and it is lost. For this reason, it may be said that heat is the ultimate form into which all other forms of energy tend to be transformed. Heat, being a form of energy, may be converted by suitable appliances into mechanical work. If the number of available heat units is known, it is possible to calculate with exactness the mechanical work that should be obtained for that quantity of heat. Energy may readily be converted from one form to another, but can neither be created nor destroyed; this is known as the law of the conservation of energy. If the heat is fully utilized, the mechanical work produced is equivalent to the quantity of heat expended.
65. The first attempt to determine, by experiment, the number of work units equivalent to one heat unit, known as the mechanical equivalent of heat, was made by Joule. The apparatus used consisted essentially of a box containing water which was stirred by a set of paddles kept in motion by weights attached to cords running over pulleys. The agitation of the water resulted in a rise in its temperature, and, losses by friction and radiation having been properly allowed for, the heat given up to the water, as indicated by the rise in temperature, was the equivalent of the work done by the weight in descending. As a result of these and similar experiments, the mechanical equivalent of the British thermal unit is now generally taken as 778 foot-pounds.
The foot-pound is a unit by which work is measured. It is the work expended in raising a weight of 1 pound vertically through a distance of 1 foot. The work, in foot-pounds, done by lifting a body may be found by multiplying
