or about 13.8 per cent. gives the combined efficiency

Now, if the student will sum up the total heat losses per minute (in this example this sum=158210 B.Th. U.), and add the heat equivalent of the useful work done (25390 B.Th. U.), the result will equal the heat of combustion (183600 B.Th. U.), for in any heat installation the

FIG. 152.

Electrical Horse-Power.-Electrical energy is developed chiefly by the use of steam engines driving electrical machines or dynamos producing a potential difference at the dynamo terminals, resulting in what is called a current of electricity. The dynamo therefore is a machine for the production of electrical energy by the expenditure of mechanical work, the former being measured by the product of the current produced (amperes) and the difference of potential in volts at the terminals. The unit of electrical energy, known as "the watt," is the rate of doing work in a circuit by 1 watt in 1 second. Thus the power developed by a dynamo giving 20 amperes at 100 volts to a circuit is 20 × 100=2000 watts, or, taking 1000 watts as 1 kilowatt (written 20 × 100 "kwtt." for short) = 2 kwtts. The connection between 1000

electrical and mechanical energy is given thus:—

(The student should here refer back to the notes on p. 195.)

=

Thus a dynamo giving 50 amperes at 200 volts is supplying 50 x 200 = 10 kwtts. ; or an amount of electrical energy equivalent 1000 to 10 x 134 134 mechanical horse-power. To generate this amount of electrical energy the dynamo will require to be supplied with an excess of mechanical energy over 13·4 H.P., due to various losses, including those of friction. The efficiency of a dynamo varies very much according to its size, whereas large machines of 100 kwtts. and over have an efficiency of 90 to 92 per cent.; smaller machines (of, say, 5 to 10 kwtts.) have an efficiency of only 80 to 85 per cent. The efficiency here referred to is the ratio of electrical energy developed to the mechanical energy expended. Thus, the previous dynamo giving 10 kwtts., equal to 134 electrical H.P., might require, say, 16 H.P., as measured by a dynamometer on the engine shaft. In that case, its efficiency would be

13.4
× 100=83.75 per cent.
16

If E.H.P. represents Electrical Horse-Power as registered by the voltmeter and ammeter (kwtts. × 1·34), and B.H.P. represents the Brake Horse-Power of the engine (if direct coupled), or mechanical horse-power transmitted by the belt or ropes, then

A number of practical exercises on the work dealt with in this chapter will be found on pp. 349 to 352.

CHAPTER XVI.

COMPARATIVE COSTS OF POWER PRODUCTION.

Various Methods of Power Production.-There are several ways in which power or mechanical energy can be producedsteam, gas, oil, electric, hydraulic, etc.; and in order to compare the various systems, some unit of measurement is required on which to base the calculations, which shall take into account not only the different conditions under which the engine or motor may be working, but also the variations in the cost of the medium employed. For example, with steam engines and other heat motors generally, we measure the number of heat units used by the engine or motor per I.H.P. or per B.H.P. per minute, as recommended by a Committee of the Institution of Civil Engineers some few years back. Such a figure is extremely useful, and comprehensive in its character, being quite independent of the medium employed, whether it be steam (saturated or superheated), gas (lighting or power), oil, or hot air; and it possesses a number of advantages over the old method of expressing the result of steam engine tests in lbs. of coal per I.H.P. per hour, or lbs. of steam used per I.H.P. per hour, as these latter expressions are incomplete without knowing the quality of the coal used, the kind of steam produced, and the pressure at which it is generated. The weight of steam used per H.P. per hour in an engine using saturated steam is not comparable with that used by an engine working with superheated steam, owing to the increased quantity of heat per lb. which the latter medium contains. Neither can the weight of best steam coal used by an engine and boiler be compared with the weight of coal-dust, coke, breeze, or other inferior fuels, without knowing the heating value of the fuel employed. Consequently there arose a necessity for some term which would, in its expression, combine the varying heating powers of different fuels, and heat-capacity of different qualities of steam. Also the adoption of such a term as thermal efficiency for the comparison of engine performances lends itself to comparing the steam engine with the gas engine, oil engine, or other energy motor.