REGISTRATION BY PHOTOGRAPHY 65 



The spark is produced by a small induc- 

 tion-coil, known as du Bois-Rey moneys chariot 

 apparatus ; it strikes between two blunt points 

 of platinum-iridium, carefully machined and 

 polished. These are fixed to the two jaws of a 

 pair of wooden pliers, which tend to close by 

 elasticity, and are kept apart by a micrometer 

 screw. At a distance of about 2 cms. from the 

 spark, and facing the plate, a plate of ground 

 glass is fixed. As I have previously mentioned, 

 the light of the spark produces on this glass an 

 extensive luminous patch, much easier to ob- 

 serve than the naked spark, and giving on the 

 photographic plate impressions of much more 

 regular form. The regulating of the spark is 

 the delicate part of the experiment. The 

 induced current must first be adjusted, by 

 modifying the primary current on the one 

 hand, and the position of the secondary coil 

 on the other, till the spark becomes very small. 

 The points are washed in alcohol, then a slip of 

 dry paper is drawn between them, for the 

 purpose of drying and repolishing their surface ; 

 then the micrometer screw is turned so as to 

 make the spark as short as possible, yet without 



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