of the Law of Efficiency of an Electric Motor, 125 



current will be, and what the energy of it ; also what the work 

 done by the motor is. 



First complete the construction as follows : — Through F 

 draw F Gr H parallel to B C, and through G draw KGL 

 parallel to A B. Then the actual electromotive force at work 

 in the machine producing a current is E — e, which may be 

 represented by any of the lines A F, KG, GH, or L C. 

 Now the electric energy expended per second is EC ; and 



since 0=^, E(E _ g) 



2R ; 

 and the work absorbed by the motor, measured electrically, is 

 e(E-e) 



XR being a constant, the values of the two may be written 

 respectively 



E(E-«) 

 and 



e(E-e). 



Now the area of the rectangle 



AFHD = E(E-e), 

 and that of the rectangle 



GLCH = e(E-e). 



The ratio of these tico areas on the diagram is the efficiency of a 

 perfect motor, under the condition of a given constant electro- 

 motive force in the electric supply. 



(2) So far we have assumed that the efficiency of a motor 

 (working with a given constant external electromotive force) 

 is to be measured electrically. But no motor actually converts 

 into useful mechanical effect the whole of the electric energy 

 which it absorbs, since part of the energy is wasted in friction 

 and part in wasteful electromagnetic reactions between the 

 stationary and moving parts of the motor. If, however, we 

 consider the motor to be a perfect engine (devoid of friction, 

 not producing wasteful Foucault currents, running without 

 sound, giving no sparks at the collecting-brushes, &c), and 

 capable of turning into mechanical effect 100 per cent, of the 

 electric energy which it absorbs, then, and then only, may we 

 take the electrical measure of the work of the motor as being 

 a true measure of its performance. Such a " perfect " elec- 

 tric engine would, like the ideal "perfect" heat-engine of 

 Carnot, be perfectly reversible. In Oarnot's heat-engine it is 



