PHOTOGRAPHING ELECTRICAL DISCHARGES. 307 



plate. Such a plate consists of a sheet of glass coated over with 

 bromide of silver incorporated in a film of gelatin. When single 

 positive or negative discharges are desired, the photographic plate, 



Fig. 2. A Negative Discharge. 



the back of which is covered with tin foil, is placed between the 

 two discharging points, and the strength of the discharge is regu- 

 lated so that, while it shall cover as much of the plate as pos- 

 sible, it shall not spread over the edge. The tin foil performs a 

 double function : it does away with the spark on the back of the 

 plate, which would otherwise occasion a confused figure ; and it 

 also assists in causing a uniform spreading out of the discharge. 

 It does not seem to modify the character of the figure produced in 

 any way. It, however, affects its form, since, if the foil be smaller 

 than the plate, the discharge seems to have a great disinclination 

 to extend itself beyond the edge of the foil, but would even curve 



