80 THE HUMAN MOTOR 



outputs of the various pieces ot apparatus thus energised, and 

 also all the energy lost in the various conversions and in the various 

 electrical circuits, we should find that this sum was equal to the 

 original input. 



The proportion of the total input which is available for useful 

 work depends on the efficiency of the system. In some cases 

 the whole of the output is deliberately wasted as in a Prony 

 brake ( 39) utilised in testing the mechanical output of an 

 engine where the output is dissipated in friction. 



In an inanimate heat engine, the output of work depends on 

 the quanity of heat supplied by the fuel. If no fuel is supplied, 

 no work can be done. The animal motor can., however, do work 

 whether well or ill nourished. 



In any motor the total expenditure, or input, of energy may be 

 divided into the static expenditure D*, needed to overcome the 

 forces of friction, etc., and the dynamic expenditure D^ which 

 corresponds to the useful work done. 



In inanimate machines D s is small compared with D</. But 

 this is not the case in the animal motor, which is always at 

 pressure, because if it ceased an instant to be in that state, life 

 itself would be arrested. Its static expenditure constantly takes 

 place, and is permanent, because it is determined (it is the 

 essential trait that has already been noted) by physiological 

 necessities. According to Wilhelm Ostwald, " the proper work- 

 ing and healthiness of the organs depends on the constancy of 

 the temperature " (in the higher animals). And as this is a 

 question of internal work ( 12) all the energy expended statically 

 will be dissipated in heat. 



The work done is only a portion, the rest degrades into heat. 

 Thus in very excessive work there may be such a production of 

 heat that the animal might suffer from it. Animals that are 

 forced to race die by excess of " thermogenesis," according to 

 Chauveau.f 1 ) 



61. The Yield of Motors. The relation of useful energy of a 

 motor to the energy that it expends is its yield. In para. 58 

 it was shewn that : 



the yield or the co-efficient of useful effect being always less 

 than unity. Motors can therefore be compared with each other, 

 either according to their power or their yield. The useful 



(*) A Chauveau (special edition of four notes which appeared in Comptes 

 Rendus A cad. Sciences, 1907, supplemented by an explanatory note 

 published by Gauthiers-Villars) . 



