EXAMPLE—An incandescent lamp taking one-half an ampere of current on a circuit having a pressure of 100 volts, or a lamp taking one ampere on a circuit having a pressure of 50 volts, would each be consuming 50 watts of energy, and this multiplied by the number of hours would give the total number of watt-hours for any definite time.
The watt, then, is an accurate and complete unit of measurement and is generally applicable to all forms of electrical consumption.
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.
Ques. How is heat measured?