442 



PROCEEDINGS OF THE AMERICAN ACADEMY 



TABLE VII. 



38 ohms, the electro-motive force rapidly increases by an amount which 

 is approximately given by the formula, E-= S (.042 — .0009 R), from 

 which we see that the electro-motive force continually increases as we 

 diminish the resistance, and, if the resistance could be reduced to zero, 

 would attain the value J5^ = .042 S. 



The column E^ is computed by assuming the electro-motive force 

 proportional to the velocity. This column can be used more con- 

 veniently than that marked E, since with small resistances the power 

 required was so great as to make the belts slip, and greatly diminish 

 the speed. 



In Table II. some measurements of the power are given, as also the 

 ratio of the theoretical power to that actually employed. The latter was 

 measured by the dynamometer, the former computed by the very con- 

 venient theoretical formula, IF= f CE. From the results, it will be seen 

 that, for large resistances, the power employed, beyond that required to 

 drive the machine, is insignificant, but rapidly increases as the resistance 

 diminishes ; the efficiency also at the same time increasing and attaining 

 its greatest value with the smallest resistances. Of course, the absolute 

 efficiency, or ratio of electricity generated to power expended, would 

 be still less than this, being very small for large resistances, and attain- 

 ing a maximum of about 30 per cent. AVhen we consider, however, 

 how large an amount of work is consumed by even a small amount of 

 heat, the coefficient in the above cases must be regarded as large. 



A series of experiments was next made to determine the strength 

 of the current generated in different positions of the armatiu-e. The 

 apparatus was constructed by Mr. S. J. Mixter, and consisted of a 

 wooden wheel attached to the armature, and revolving with it. On this 

 rested a brass wire ; and a strip of copper was inserted in the wheel, so 

 that it established contact between the axle and the wire, through an 

 angle of about lO**. The latter was supported by a second larger 



