of the Law of Efficiency of an Electric Motor. 131 



It is at* once evident that the power transmitted is increased 

 fourfold, while the efficiency remains the same. If we increase 

 the number of coils, as before, on the machine at the receiving 

 end of the line so as to bring up the difference B — e to the 

 value it had in fig. 6, the scale of the diagram will still be 

 unaltered; the power transmitted will be now only double 

 instead of quadruple; but the efficiency will thereby be more 

 than doubled, the heat-waste being the same, and the energy 

 utilized more than twice as great. High electromotive force 

 is therefore advantageous in both cases, especially in the case 

 of a great resistance in the line. 



(8) It only remains to point out a curious contrast that 

 presents itself between the efficiency of a perfect heat-engine 

 and that of a perfect electric engine. We saw (§ 2) that the 

 one could be expressed as a function of two temperatures, 

 whilst the other could be expressed as a function of two elec- 

 tromotive forces. But in the heat-engine the efficiency is the 

 greatest when the difference between the two temperatures is 

 a maximum; whilst in the electric engine the efficiency is the 

 greatest when the difference between the two electromotive 

 forces is a minimum. The two cases are contrasted in figs. 9 and 

 10, fig. 9 showing 



the efficiency of a Fig. 9. Fig. 10- 



heat-engine work- 

 ing between tem- 

 peratures T and t 

 (reckoned from ab- 

 solute zero) ; whilst 

 fig. 10 shows the 

 efficiency of an 

 electric engine re- 

 ceiving current at 

 an electromotive 

 force E, its counter 

 electromotive force being e. Joule's remark, here illustrated, 

 that^ an electric engine may be readily made to be a far more 

 efficient engine than any steam-engine, is amply justified by 

 all experience. _ But in spite of this fact, electric engines are, 

 as yet, dearer in practice than heat-engines, simply because 

 energy in the form of electric currents supplied at a high 

 potential is, as yet, much more costly to produce than energy 

 in the form of heat supplied at a high temperature. 



Energy 

 washed 



Energy 

 utilised 



