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following work the air gap was 49 mm. and the current 22 amperes. In 
order to obtain a photograph of the current which passed between the 
points transverse to the magnetic lines of force it was necessary for the 
Fig. 7. 
light from the pinhole to pass through the hollow cores of the magnet. 
This was accomplished by fitting the two portions of the tube against 
the magnet, as shown in photograph, Fig. 8. An auxiliary wooden tube 
25 cm. square and 12.5 cm. long was placed between the coils of the 
