248 



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plate was sufficiontly exposed. In case of the 

 spark discharge which fogged the plate if ex- 

 posed too long, the cardboard was first removed 

 and tlie exposure made by withdraw'ing the 

 slide. 



The magnet used was of the Faraday type 

 (photographs, Figs. 7 and 8), with pole pieces 

 3* inches in diameter, and with a current of 22 

 amperes gave, midway between the pole pieces, 

 when 49 mm. apart, a field strength of 6,400 

 gausses. Longitudinally through the cores and 

 the pole pieces was a hole 2.54 cm. in diameter. 

 If the holes Wiere filled by placing in them an 

 iron cylinder of tlie same material as the cores, 

 and the air gap reduced to 1 mm., a field 

 strengtli of 40,000 gausses for a current of 44 

 amjicres conld be jiroducc^l. In most of the 



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