340 ELEMENTARY LESSONS ON [CHAP. vn. 



of the energy of the current will be turned into useful 

 work. If it be set to run very quickly, so as to generate 

 a considerable back-current, it will utilise a larger pro- 

 portion of the energy of the direct current, but can only 

 run fast enough to do this if its load be very light. 

 Jacobi calculated that the practical efficiency lay between 

 these two extremes, and that an electromotor would turn 

 the energy of a battery into work in the most effective 

 way when it was allowed to do its work at such a speed 

 that the battery current was thereby reduced to half its 

 strength. This is indeed true if it be desired to do the 

 work at the quickest possible rate. But where economy 

 in working is desired, and when it is not needful to get 

 through the work as rapidly as possible, or to consume 

 materials in the battery at a great rate, then a higher 

 economic efficiency will be attained by making the electro- 

 motor do lighter work and spin at a greater speed ; for 

 then the back-current may be made nearly equal to the 

 original current, and the original current be reduced 

 thereby: to a small fraction of its strength. The materials 

 of the battery will be more slowly used, and it will take 

 a longer time to do the total amount of the work, but 

 the percentage of energy of the currents turned into work 

 will be higher. A Siemens dynamo -electric machine, 

 used as a motor, can attain an efficiency of over 85 per 

 cent. 



378. Cost of Working. The cost of working 

 electromotors by batteries is great. A pound of zinc 

 contains only about \ as much potential energy as a 

 pound of coal r and it costs more than twenty times as 

 much : the relative cost for equal amounts of energy is 

 therefore about 120 : i. But, as shown above, an electro- 

 magnetic engine will turn 85 per cent of the electric 

 energy into work, while even good steam-engines only 

 turn about 10 to 20 per cent of the energy of their fuel 

 into work, small steam-engines being even less efficient. 

 But, reckoning electromagnetic engines as being 5 times 



