346 FRAGMENTS OF SCIENCE 



a circle about tliirty feet in diameter. As tlie velocity of 

 rotation augmented, these images blended into a continu- 

 ous ring of light. At a particular instant the electro- 

 magnet was excited, currents were evolved in the rotating 

 cube, and the strength of these currents, which increases 

 with the conductivity of the cube for electricity, was 

 practically estimated by the time required to bring the 

 cube and its associated mirrors to a state of rest. With 

 bismuth this time amounted to a score of seconds or more: 

 a cube of copper, on the contrary, was struck almost in- 

 stantly motionless when the circuit was established. 



