60 Trans. Acad. Sci. of St. Louis. 
grounded on an independent ground. ‘The diagonal 
brushes of the influence machine were also grounded in- 
dependently of all other ground contacts. 
Conditions Which Precede Disruptive Discharge. 
In making an examination of the conditions which pre- 
cede a disruptive discharge, the arrangement shown in 
Fig. 1 was somewhat modified. The metal plate was re- 
moved. The photographic plate was lifted above the 
glass top of the table, and supported at its edges only by 
insulating supports. The pin-head terminals were moved 
nearer together and rested on the same film, which had 
dimensions 314x414 inches. The spark gap at the ma- 
chine was reduced to 3 or 4mm. ‘The plates of the ma- 
chine were revolved very slowly. Several small sparks 
at the machine having followed each other in quick suc- 
cession, a spark would pass across the film between the 
pin-head terminals. In a series of plates to be described, 
the exposure was in the negative line. The positive 
terminal was grounded, on an independent ground. 
Fig. A of Plate XX shows the effect of a single small 
spark at the machine. The negative inflow at the pin- 
head terminal marked G, which was connected with the 
ground is very slight. In B and ©, the exposure was 
slightly increased. This may be done by increasing the 
number of minute sparks at the machine, or by slightly 
increasing the length of this spark. Fig. A and Fig. B 
of Plate XXI show the result of a slight increase of the 
spark length. The ionization of the air due to the inflow 
of negative electricity to the grounded pin-head has in 
Fig. A extended to the small fogged area around the pin- 
head, which is receiving the negative discharges from 
the machine. This is the negative glow. We have here 
the same phenomenon that was pointed out in the former 
paper. It is apparently with difficulty that the negative 
discharge can be forced out of the negative terminal, but 
it can easily be drawn in from the film to the positive 
terminal. The results here shown were obtained when. 
