MAGNETISM. 151 



with the thumb and finger, fine iron filings upon the 

 glass. The filings will arrange themselves in the 

 familiar lines called the magnetic phantom, and the 

 whole being magnified to ten feet or more in diameter 

 makes a very striking picture. 



5. The elongation of an iron rod when strongly 

 magnetized, may be shown by placing a small helix 

 around the iron rod of the common pyrometer made 

 for showing the longitudinal expansion of a rod by 

 heat. To the end of the index finger that sweeps over 

 the quadrant affix a small bit of plane mirror not more 

 than one fourth of an inch square. So adjust the light 

 to this small mirror that the reflection from the latter 

 will fall upon the most distant part of the room ; the 

 farther away the better. When the current of elec- 

 tricity is sent through the helix the rod will be slightly 

 elongated, but the slight tilting of the mirror may 

 become a displacement of two or three inches at a 

 distance of thirty feet. 



DIAMAGNETISM. 



The electro-magnet 'for demonstrating diamagnetic 

 phenomena need not be over three or four inches in 

 length, and the poles an inch apart. 

 Objects to be tested may be suspen- 

 ded by a thread between the poles, 

 and the whole projected either in a 

 beam of parallel rays or in front of 

 the focus of a lens. In the latter 

 case the whole will be seen in pro- 

 file, but perfectly distinct. The fol- 

 lowing experiments may be projected 

 with such a magnet if a battery of ~ Fig. 

 three or four cells be used : 



