RESISTANCE OF MATERIALS 81 



energy is, in practice, that which is actually employed, the sterile 

 energy T t -, comprising all the mechanical and calorific loss which 

 occurs in transmission from the motor to the receiver. From 

 this point of view, the two terms of the yield are T M and D, D 

 being the total expenditure of the motor, static and dynamic : 



This is the gross yield : 



D s + Dj 

 Industry, as a rule, is only concerned with the value r. 



But this co-efficient must be analysed. The motor, which has 

 an expenditure D, produces work greater than T u ; this inta 

 heat engine is called the indicated work. 



The machine, owing to internal resistances, has only given a 



T 



fraction =-, of the indicated work, this being its yield. The 

 A* 



work lost absorbs (T T w ), and the relation of T to D will be 

 the co -efficient of transformation of the energy. Also in actual 

 work the motor only expends DJ ; the net yield being the value : 



R-3L-. 



D, 



~D S is considerable in the case of animated motors, therefore it 

 must be deducted from the total expenditure D to know the net 

 yield of the muscle, and if, in each method of work, the value of 

 the organic yield is taken, the true co-efficient of the transforma 



T 



tion of muscular energy ~ can be deduced from it. 



62. The idea of the yield was only recently defined. In 1819 

 Navier wrote : "A machine is all the more perfect the nearer its 

 useful result (T*) is to the quantity of the energyt 1 ) that it con- 

 sumes "( 2 ). Coriolis is more precise in 1829 : "When one wishes 

 to give an idea of the efficiency of a machine . . . one compares 

 what it yields with what it receives. The fraction that expresses 

 the relation between these two quantities is the measure of the 

 degree of perfection of the machine." ( 3 ) 



Carnot( 4 ) in 1824, developed the theory of the cycle which bears 

 his name, and showed that the yield, in such a cycle (it is known 

 that thermodynamic operations, in such a case, are closed and 

 reversible ( 35)) is the maximum. This cycle, which assumes, 



( x ) Quantity of action was then synonymous with work. 



( 2 ) Navier, Notes, p. 382, in Architecture Hydraulique, by Belidor. 



( 3 ) Coriolis, loc. cit., p. 131. 



( 4 Sadi-Carnot, Reflexions sur la puissance motrice du feu (I.e.). 



