1885.] NEW YOKK ACADEMY OF SCIENCES. 93 



Of the total iiiechaiiical ])o\ver applied, about one per cent was 

 lost in friction of the armature shaft, and resistance of the air to 

 its rapid revolution. 



Two and one-half per cent only remains to be accounted for, 

 and were presumably lost in the form of heat and eddy currents. 



Every precaution was taken to avoid results which would not 

 appear in every-day use, and all of the machines were run under 

 full load for ten hours before the measurements began, and so 

 were at as high a temperature as would be reached in actual 

 practice with the same atmospheric temperature. The perform- 

 ance of this particular machine only exceeded the least efficient 

 of the machines tested by 2^^^ per cent total efficiency. 



These results show that this high efficiency is not extraordin- 

 ary, but is, and should be attained by all dynamo makers build- 

 ing similar types. 



In the case of the Weston (7 M.) dynamo, already specified, 

 the power applied was distributed as follows in the first full load 

 test : 



Friction and wind resistance of armature. .0106 Total. 

 Electrical energy lost as heat in armature. .0559 " 



" " " in creating field 0170 " 



" " in external circuit.. .8992 " 



Total of power accounted for 9827 



Electrical energy lost in eddy currents, 



heat, and otherwise unaccounted for. .. .0173 



Total power as per dynamometer 1.0000 



This differs from the average already quoted, because slight 

 yariations of the conditions would cause any of the machines to 

 vary somewhat in their percentages. 



The greatest cause of uncertainty in experiments heretofore 

 made upon the transformation of mechanical power, has been our 

 lack of certainty of accuracy in the measurements of the me- 

 chanical power driving the machine. The dynamometer must 

 sum up the whole power yielded to the dynamo with as great ac- 

 curacy as is possible for all other measurements in part. 



This dynamometer must be capable of being standardized by ab- 

 solute measurement, and, after being standardized, the machines 

 to be tested must be able to be attached to it or removed from it 

 without altering the centres oradjustmentsof the dynamometer. 

 It must be of great sensitiveness to small variations of load while 

 measuring large amounts of power with great steadiness. 



All of these conditions were fulfilled by the dynamometer in- 

 Yented by Mr, Wni. P. Tatham, President of the Franklin In- 



