RESISTANCE OF MATERIALS 79 



T/ 



and =r the co-efficient of loss. Perpetual motion would take place 



JLmv 



T 

 if f*r^~ 1, which is impossible. 



*-m 



59. Power of Motors. The power of a motor is the quantity 

 of work or mechanical energy developed in unit time. (It is 

 unfortunate that the term "power" is confused with "force.") 

 Various motors can be compared by the power which they can 

 exert continuously. The practical unit of power is the poncelet, 

 which develops 100 kilogrammetres per second, but the more 

 generally used unit is the horse power " cheval-vapeur " (symbol 

 H.P.) which is 75 kilogrammetres. The watt is also employed 

 for electric motors, the watt being a unit of electric power 

 equalling approximately '102 kilogrammetres ; the kilowatt (1,000 

 watts) corresponding to a power of 102 kilogrammetres (see 

 units 63). One horse power equals 735'75 watts. 



In live motors, the work is not continuous on account of the 

 phenomena of fatigue (S 171). It is necessarily intermittent since 

 a day's work is interrupted by a certain number of halts 

 or intervals of repose. Taking the total of the work done, and 

 the total duration of the periods of activity, the quotient of 

 these two quantities gives the useful power of the man or the 

 animal. The power is the rate production of energy. 



In industrial practice it is easier to record the quantity of work 

 in a day, halts and repose included. 



To increase the power of ordinary motors it is necessary to 

 increase their size. Hence we may consider power per unit of 

 weight. Steam engines develop one horse power per 100 kilo- 

 grammes of weight. Internal combustion engines give a horse 

 power per 12, 7, 6 and even 3' 500 kilogrammes, and there is 

 included in that weight the cylinders and their accessories 

 (carbureter, oil pump, magneto). These results have been 

 attained mainly by high speeds of rotation. 



60. Expenditure of Motors. The expenditure or input of a 

 motor is its consumption either of fuel in inanimate motors, or 

 of nourishment in animated motors. Ever}' machine or system 

 of machines transforms, but does not create, energy. These 

 transformations may be numerous. Take, for example, the 

 case of electrical supply. The thermal energy of the coal burnt 

 under the boilers is (with considerable loss) converted into 

 mechanical energy in the steam turbine or reciprocating engine. 

 This mechanical energy is converted into electrical energy in the 

 dynamo. This may be again converted into mechanical energy 

 by means oi electric motors, into thermal energy in electric 

 radiators or furnaces, into chemical energy in electrolytic pro- 

 cesses, and so forth. If we could accurately measure both the 



