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 IL. 
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 I, 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- 
