[Illustration: FIG. 85.--Tyndall’s experiment illustrating the production of heat by friction. A brass tube about 7 inches in length and 3/4 of an inch in diameter, is fixed on a small wheel. By means of a cord passing round a much larger wheel, this tube can be rotated with any desired velocity. The tube is three parts full of water, and is closed by a cork. In making the experiment, the tube is pressed between a wooden clamp, while the wheel is rotated with some rapidity. The water rapidly becomes heated by the friction, and its temperature soon exceeds the boiling point. The pressure caused by the formation of steam forces out the cork and projects it to a height of several yards.] A watt of electrical energy corresponds to 1/746 of a horse power of mechanical energy; hence, if a lamp or motor require energy equivalent to 1/746 of a horse power for one hour, it might be said to take one watt-hour. =Mechanical Equivalent of Heat.=--The eminent English physicist, James Prescott Joule, worked for more than forty years in establishing the relation between _heat_ and _mechanical work_; he stated the doctrine of the conservation of energy and discovered the law, known as Joule’s law, for determining the relation between the heat, current pressure, and time in an electric circuit. =Ques. What is heat?= Ans. A form of energy. Heat is produced in the agitation of the molecules of matter--the energy expended in agitating these molecules is transformed into heat. [Illustration: FIG. 86.--Joules’ experiment on the mechanical equivalent of heat, in which he caused paddle-wheels to rotate in a vessel of water by means of falling weights _W_. The amount of work done by gravity upon the weights in causing them to descend through any distance _d_ was equal to their weight _W_ times the distance. If the weights descended slowly and uniformly, this work was all expended in overcoming the resistance of the water to the motion of the paddle-wheels through it; that is, it was wasted in eddy currents in the water. Joule measured the rise in the temperature of the water and found that the mean of his three best trials gave 427 gram meters as the amount of work required to develop enough heat to raise a gram of water one degree. He then repeated the experiment, substituting mercury for water, and obtained 425 gram meters as the work necessary to produce a calorie of heat. The difference between these numbers is less than was to have been expected from the unavoidable errors in the observations. He then devised an arrangement in which the heat was developed by the friction of iron on iron, and again obtained 425. This corresponds to 772 foot pounds, but later experiments show that the correct value is 778 foot pounds.] =Ques. How is heat measured?= Ans. In British thermal units (B.t.u.). =Ques. What is the British thermal unit?=