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 3 1 / 4x4 1 / 4 inches. The spark gap at the ma- 

 chine was reduced to 3 or 4 mm. 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 C, 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 



