245 
vase of the 
spark discharge which fogged the plate if ex- 
plate was sufficiently exposed. In 
posed too long, the cardboard was first removed 
and the exposure made by withdrawing the 
slide. 
The magnet used was of the Faraday type 
(photographs, Figs. 7 and 8), with pole pieces 
34 inches in diameter, and with a current of 22 
aimperes 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 were filled by placing in them an 
iron cylinder of the same material as the cores, 
the a field 
strength of 40,000 gausses for a current of 44 
sunperes could be produggik In 
and air gap reduced to 1 mm., 
most of the 
F/ee a Va 
