Ans. The quantity of heat required to raise the temperature of 1 lb. of pure water 1° Fahr., at or near 39.1° F., the temperature of maximum density. =Ques. What is the mechanical equivalent of heat?= Ans. The number of foot pounds of mechanical energy equivalent to one British thermal unit. Joule’s experiments 1843-50 gave the figure 772 ft. lbs. which is known as Joule’s equivalent. Later experiments gave higher figures, and _the present accepted value is 778 ft. lbs._, that is: 1 B.t.u. = 778 ft. lbs. =Electrical Horse Power.=--It is desirable to establish the relation between _watts_ and _foot pounds_ in order to determine the _capacity_ of an electric generator or motor in terms of _horse power._ One watt is equivalent to one joule per second or 60 joules per minute. One joule in turn, is equivalent to .7374 ft. lbs., hence 60 joules equal: 60 × .7374 = 44.244 ft. lbs. Since one horse power = 33000 ft. lbs. per minute, the electrical equivalent of one horse power is 33000 ÷ 44.244 = 746 watts. or, 746 / 1000 = .746 kilowatts (K.W.) Again, one kilowatt or 1000 watts is equivalent to 1000 ÷ 746 = 1.34 horse power. =The Farad.=--The measure constructed to hold a gallon of water may be called the gallon measure. The capacity of a condenser which would contain a charge of one coulomb under one volt pressure is the _farad._ It may seem strange that there is a unit of quantity and another of capacity to hold that quantity, when in the case of water the term “gallon” may suffice for the measure and the liquid it can hold. Electricity in this respect, however, corresponds to a _compressible fluid_ or a _gas_. A gallon measure may hold a gallon of gas or ten; it depends entirely upon the pressure. So a condenser of a certain size may hold any number of coulombs, according to the electrical pressure. The farad being inconveniently large for practical use, one-millionth of a farad, called a _microfarad_, is generally adopted. CHAPTER VIII EFFECTS OF THE CURRENT The term “electric current,” in the present state of our knowledge, should be regarded as denoting the existence of a state of things in which certain definite experimental effects are produced, for some of which there certainly is no analogy exhibited in ordinary hydraulic currents. The following are the most important of these effects: 1. Thermal effect; 2. Magnetic effect; 3. Chemical effect. It is rather to these effects than to any imaginary current flow in the conductor that the mind of the reader should be directed. With this preliminary caution, which should never be lost sight of, the use of familiar words and expressions connected with the flow of water in pipes is justified in order to avoid roundabout and cumbrous phrases which, though perhaps more nearly in accord with present knowledge of the facts, would not tend to clearness or conciseness.