CHAPTER II. 

 YIELD OF THE HUMAN MACHINE. 



129. Definition. The yield is the relation of the mechanical 

 energy produced, to the energy expended. It has been explained 

 that the industrial yield, r, is a gross value deducted from the 

 relation of useful energy to the total energy (static and dynamic) ; 

 whilst the net yield R only includes the dynamic expenditure. 

 Then (see 61) ": 



T T 



- D, + D rf : D; 



Example : An adult expends 100 calories per hour in repose, 

 performing work to the amount of 25,500 kgm (i.e. 60 calories per 

 hour), he expends altogether 340 calories. Therefore : 



D, = 100, T = 60, D rf = 240, and 



If the value of the net yield of the subject is once definitely 

 determined the amount of his expenditure will give the value of 

 the work performed : 



T = R x D rf . 



To obtain D^ the consumption of oxygen before the trial, and 

 afterwards, when normally working is measured. From the 

 ratio of 4-90 Calories per litre (at 0C and 760 millimetres 

 of mercury barometric pressure) can be calculated the energy 

 expended T> s and D^ in a period of time or a few minutes (thirty 

 minutes in the course of work). 



This method of operation is the only correct one, seeing that the 

 initial state of the subject is always difficult to define. By coming 

 to the workshop in a certain state of fatigue, the workman has 

 already a high static expenditure. It is therefore impossible 

 to take his gross expenditure D^ + Dj as a basis from which 

 to calculate the amount of work he performs, in other words, 

 r is very variable, much more so, as will be shown later, than the 

 net yield R. 



Before proceeding further, the terms whose quotients give the 

 net and gross yields must be accurately defined. First of all 

 there is the useful mechanical work, the industrial effect, a quan- 

 tity which can generally be calculated without difficulty. The 

 work done in ascending a mountain, a staircase, or a ladder is 

 the product of the weight of the body (with or without a load) 



