ELEMENTARY LESSONS ON [CHAP. x. 



furnished by friction machines and by Leyden jars (see 

 Lesson XXIII. on Phenomena of Discharge). 



Fig. 149. 



For studying discharge through glass vessels and tubes 

 from which the air has been partially exhausted, the 

 coil is very useful. Fig. 150 illustrates one of the 

 many beautiful effects which can be obtained, the spark 

 expanding in the rarefied gas into flickering sheets of 

 light, exhibiting striae and other complicated phenomena. 

 A 4O1. Currents Induced in Masses of Metal. 

 A magnet moved near a solid mass or plate of metal 

 induces in it currents, which, in flowing through it from 

 one point to another, have their energy eventually 

 frittered down into heat, and which, while they last, 

 produce (in accordance with Lenz's law) electromagnetic 

 forces tending to stop the motion. Several curious 

 instances of this are known. Arago discovered that 

 when a disc of copper is rotated in its own plane under 

 a magnetic needle the needle turns round and follows 

 the disc ; and if a magnet is rotated beneath a balanced 

 metal disc the disc follows the magnet. Attempts were 

 made to account for these phenomena known as 



