Rotation of the Plane of Polarization of Light. 173 



rate of speed would have increased, to a certain extent, the cur- 

 rents 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 ma- 

 chine, and is not the case with the Siemens and the Gramme 

 machine. A certain proportion between the resistance of the 

 machine and that of the outside circuit is undoubtedly best 

 for greatest efficiency of dynamoelectric machines ; and a cer- 

 tain velocity is necessary to attain the greatest efficiency. 



From my experiments, I should class the machines as fol- 

 lows : — 



Gramme, 



Siemens, 



Wilde. 



Theoretically the Siemens machine should give the best 

 results. At the time of my experiments the Siemens machine 

 suffered the disadvantage 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 connexion with electric lighting ; and a suitable elec- 

 trodynamometer 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 department. 



XXX. A Proof of the Electromagnetic Rotation of the Plane 

 of Polarization of Light in the Vapour of Sulphide of Carbon. 

 By A. Kundt and W. C. Rontgen*. 

 [Plate VIII. fig. 3.] 



EAEADAY, as is well known, did not succeed in demon- 

 strating the electromagnetic rotation of the plane of po- 

 larization of light in gases ; and since his time that a rotation 

 has not been observed. 



* Translated from a separate impression, with an original postscript, 

 communicated by the Authors. 



