ON CERTAIN PROPERTIES OF LIGHT- STRUCK 

 PHOTOGRAPHIC PLATES.* 



Francis E. Nipher. 



The results to be given in this paper were obtained with a 

 Topler-Holtz machine having one 24-inch plate, and with no 

 condenser attached to its terminals. The spark-length men- 

 tioned in the paper is the distance between the discharge 

 knobs of the machine. 



A parallel circuit consisting of ball-tipped brass rods about 

 six feet in length led to the insulated stool upon which the 

 photographic plate is placed for electrical exposure. A brass 

 plate a foot square was placed on the top of an insulated 

 stool, and formed one plate of a condenser. Upon this a 

 much larger glass plate is placed, upon which rests the 

 photographic plate. All of the results were obtained with 

 the Cramer "lightning' plate. Some metallic object like 

 a medal is placed upon the sensitive film, and forms the 

 other plate of the condenser. A rod about a foot in 

 length, having knobs, stands vertically over the medal. 

 The knobs of the secondary or parallel circuit are separated 

 from the plates of the condenser and from the machine 

 terminals, by small spark gaps which may be varied. Such 

 changes appear to materially affect the behavior of the 

 machine and the details of the picture produced. The rods 

 are all sleeved by glass tubing, and are then held by labora- 

 tory clamp stands. This arrangement for electrographing is 

 well known although the photographic plate has heretofore 

 been protected from the light. 



The method which has been found most convenient for 

 manipulation is to first expose the plates to the light of an 

 ordinary room for from one to nine days. A longer interval 



* Presented in abstract to The Academy of Science of St. Louis, March 19, 

 1900. 



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