Permanent Magnets. 



185 



long, and 0'55 cm in diameter. They were magnetized to satura- 

 tion in a coil, the magnetic circuit being completed by an iron 

 casting of suitable size and shape. They were then brought to 

 the permanent state by alternate heating and cooling. In both 

 ends of each, holes were bored and threads cut. The depth of 

 these holes in magnet A was 8 mm at each end. In magnet B 



o.i.6. 



300 490 CGS. 



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the hole at the south end was 11 mm deep ; that at the north 

 end 7 mm . In the experiment the magnets were placed perpen- 

 dicular to the earth's field. Pieces of brass rod, H,H,H,H, of 

 the same diameter as the magnets were screwed into their ends 

 and acted as guides for the two coils (to be described presently), 

 so that after the coils had been slipped off the magnets, they 

 could be slipped back again without trouble. DD is a brass 



-fig 



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rod about 1*5 meters long, holding two magnets together in the 

 position shown in the figure. C,C are pieces of non-conduct- 

 ing material to keep the magnets from changing temperature 

 by conducting along the rod DD. E and F are two coils of 

 very fine wire wound on paper tubes which just fit the magnets. 

 They consisted of 150 turns (five layers of 30 turns each), and 

 were about 7 mm wide. Their frames were joined by a brass 

 rod, PP, of such length that when the coil E was at the center 

 of the magnet B, the coil F was also at the center of the mag- 

 net A. By means of this rod they could be moved simultane- 

 ously over corresponding portions of the two magnets. At Gr 

 is a gauge which regulates the distance the coils are moved at 

 a time, so that as they are moved step by step from one end of 



