OF ARTS AND SCIENCES. 



443 



wooden wheel, which conkl be turned and held in any desired position 

 by inserting a pin in one of a series of holes in its circumference, at 

 intervals of 10"^. The experiment was performed by connecting the 

 brass wire and axis of the machine with the galvanometer, so that 

 durincr each revolution of the armature the current would be for an 

 instant diverted through the galvanometer, these currents following 

 each other so rapidly when the machine was running as to produce a 

 sensibly constant deflection. The larger wheel was then turned 10*, 

 and the observation repeated. The 0'^ and ISO'* of this wheel corre- 

 sj^oud to the points where the circuit is reversed by the commutator. 



TABLE VIII. 



Table VIII. gives the result of two series of experiments of this kind, 

 the wheel beinjr turned through 360° and the mean of the two read- 

 iugs at intervals of 180'' taken. Column 1 gives the angle through 

 which the wheel has been moved, and column 2 the current, the 

 main circuit having a resistance of 16.7 ohms, and the galvanometer 

 circuit a resistance of 1.3 ohms. Column 3 in like manner gives 

 the current when the resistance of the main circuit is reduced to 10 

 ohms. An examination of this table shows that the current at no 

 point becomes zero, but varies from a maximum at about 145° to a 

 minimum at 90"^. If the distance of the poles of the magnet was 

 large compared with the motion of the armature, the current would 

 vary as the sine of the angle, supposing that there was no induction or 



