132 PROCEEDINGS OF THE AMERICAN ACADEMY 



on each side of a central cast-iron wheel li" thick. The central diam- 

 eter of this wheel is 18" nearly. The whole weight of wire in this 

 machine is nearly 354 pounds." The normal velocity of machine is 

 about 600 revolutions. A greater rate of speed would have increased, 

 to a certain extent, the currents produced by the Siemens and the 

 Gramme machines : on the other hand, more horse-power would have 

 been necessary to attain this increased speed. The Wilde machine 

 requires more horse-power to run it, as the resistance of the outside 

 circuit increases. This is due to the construction of the machine, and 

 is not the case with the Siemens and the Gramme machine. A cer- 

 tain proportion between the resistance of the machine and that of the 

 outside circuit is undoubtedly best for greatest efficiency of dynamo- 

 electric machines ; and a certain velocity is necessary to attain the 

 greatest efficiency. 



From my experiments, I should class the machines as follows : — 



Gramme, 

 Siemens, 

 Wilde. 



Theoretically, the Siemens machine should give the best results. At 

 the time of my experiments, the Siemens machine suffered the disad- 

 vantage of being run at a less rate of speed than the other machines. 



I hope to pursue these tests under conditions resulting from higher 

 speed. Generally speaking, that machine is the best which gives the 

 greatest efficiency at low rates of speed ; for the necessity of high 

 speed introduces much that is detrimental to the locality of the machine 

 and to the machine itself. At the present time, alternating machines 

 are coming into notice again, in connection with electric lighting; 

 and a suitable electrodynamorneter is desirable in the measurement of 

 the current produced by these machines. The instrument which I 

 have tested above seems to fulfil the proper conditions. 



My thanks are due to the officers of the station for their generous 

 assistance and free disposal of the resources of their electrical depart- 

 ment. 



