Nipher — On the Nature of the Electric Discharge. 11 



has been charged from the negative terminal of the ma- 

 chine. 



2. By the one-fluid hypothesis. The negative dis- 

 charge flowing under compression through the wire 

 above, finds in the grounded wire below a line of leakage. 

 This ground wire greatly increases the potential drop at 

 that point. A negative discharge from the lower face of 

 the glass plate passes to the ground wire below. Simul- 

 taneously a negative discharge from the upper wire flows 

 over the film, and tends to flow downward to the ionized 

 molecules of glass in the lower face of the glass plate. It 

 constitutes a bound charge. 



The discharge effects shown may be explained by 

 either hypothesis. Plates IV and V show two plates 

 which have been exposed in precisely the same way when 

 the discharge through the wire above came from the posi- 

 tive terminal of the machine. The spark length was about 

 twice as great as the negative sparks in Plates II and III. 

 This could be done without producing sparks around the 

 plate when the ground wire was placed below the photo- 

 graphic plate. In such exposures adjustable spark ter- 

 minals a and b, Fig. 1, were used. They are so placed that 

 the negative discharge passes from a large knob to a 

 small one. In case of a reversal of the electrification of 

 the machine, the adjustable terminals are transferred to 

 the other terminal of the spark gaps. When the negative 

 discharge is being used, the positive spark gap is short- 

 ened in length to one or two centimeters. It will be seen 

 that the discharge lines in Plate IV for the positive dis- 

 charge extend outwards several centimeters from the 

 wire, while in Plate II for the negative discharge there 

 are no discharges over the film of the plate. When the 

 spark length in the negative line is made four times as 

 great as was the case in the formation of Plate II, and 

 when the ground resistance is a spark-gap of ten centi- 

 meters, the number of sparks passed through the wire 

 being increased to an hundred, the only effect is to broad- 

 en the black line shown in Plate II. There are no dis- 



