January 9, 1903.] 



SCIENCE. 



57 



chines, wheel and crauk-pin presses, lifting 

 jacks and hoists of all kinds. 



For heavy boiler work hydraulic rivet- 

 ing seems especially well adapted, as an 

 intensity of pressure can be brought to bear 

 upon the plates which is obtained by no 

 other method. 



We have already stated that compressed 

 air as now used without reheating is not 

 at all efficient as a source of motive power, 

 since the combined efficiency of compressor 

 and motor, even under favorable condi- 

 tions, is not more than 50 per cent, of 

 the available energy put into the com- 

 pressor. In other cases the efficiency is 

 as low as 20 per cent. 



In the transmission of air, within reason- 

 able limits, the loss in transmission if the 

 pipes be tight need not be considered, for 

 although there is a slight loss in pressure 

 due to the frictional resistances of the 

 pipes, yet there is a corresponding increase 

 in volume due to drop in pressure, so that 

 the loss is practically inappreciable. 



There should be no comparison between 

 the cost of power by compressed air and its 

 brilliant rival, electricity, since each has its 

 own field of usefulness, yet it may be in- 

 teresting to note for our present purposes 

 the efficiency of electric power. A modern 

 shop generator belted from an engine will 

 have an efficiency of about 90 per cent, 

 when working under favorable conditions, 

 but as the average load is ordinarily not 

 more than two thirds full load, and often 

 much less, the efficiency will not usually be 

 more than 85 per cent. Since the engine 

 friction was added to the losses in com- 

 pression, so also it should be considered 

 here, in which case the eiSeiency of genera- 

 tion will lie between 75 and 80 per cent. 

 With a three-wire 220-volt system, which is 

 very suitable for ordinary shop transmis- 

 sion when both light and power are to be 

 taken ofE the same dynamo, the loss in 



transmission need not be more than 5 per 

 cent., so that the efficiency at the motor 

 terminals will not be far from 75 per cent. 

 With motors running under a nearly con- 

 stant full load the efficiency of motor may 

 be 90 per cent. ; but with fluctuating loads 

 this may fall to 60 per cent, at quarter 

 load. In numerous tests made by the 

 speaker the average load on several motors 

 in machine shops has been only about one 

 third of the rated capacity of the motor. 

 It is interesting to note that in tests made 

 at the Baldwin Locomotive Works it was 

 found that with a total motor capacity 

 aggregating 200 horse-power, a generator 

 of only 75 kilowatts was sufficient to fur- 

 nish the current, and ordinarily only 60 

 kilowatts, or 40 per cent., was required. At 

 the present time there are in iise at these 

 works upwards of 300 motors, with a com- 

 bined total capacity of 2,200 or 2,300 horse- 

 power; whereas the generator output is 

 only about 500 kilowatts. 



Under those conditions, where the driven 

 machines are not greatly over-motored, we 

 may assume a motor efficiency of 80 per 

 cent., which may be less or greater in indi- 

 vidual eases. The combined efficiency, 

 then, of generator and motor working in- 

 termittently with fluctuating loads will be 

 about 60 per cent, of the power delivered to 

 the engine. 



For greater distances than those which 

 obtain in plants of this character the loss 

 in transmission will be greater, and higher 

 voltage must be employed in order to keep 

 down the line loss. While it is possible to 

 put in conductors sufficiently large to carry 

 the current with any assumed loss, yet the 

 cost of the line becomes prohibitive with 

 low voltage. 



Where cheap fuel is available it is found 

 in most cases that electric power can be 

 generated at the works more cheaply than 

 it can be purchased from a central station ; 



