20 Trans. Acad. Scr. of St. Lows. 
nel as it progresses towards the negative terminal. The 
system is like that of a river with its tributaries, which 
wears a channel in the earth. The channel develops and 
deepens progressively in a direction opposite to that in 
which the stream flows. It may be that such a condition 
in the air between the spark-knobs of a machine brings 
about the result which has long been known, to-wit: The 
spark length is greatest when the negative discharge 
passes from a large knob to a small knob. The diverging 
system of tributary discharge lines terminates on the 
large knob. This is readily seen by transferring the 
movable conductors, a, b, of Fig. 1, to the opposite termi- 
nals of the gaps. This arrangement is a convenient one 
for determining in a lighted room which is the negative 
terminal of the machine. 
EXPLANATION OF PLATES. 
Plate I.— Tracks of slowly moving discharges from negative terminals. 
Plate II.— Photographic plate in contact with the negative discharge 
line. 
Plate III.— Plate exposed as in II with the end of a grounded wire 
near the back of the plate. 
Plates IV, V.—Plates exposed like those of II and III to the positive 
line. 
Plate VI.— Plates exposed back to back asin V. The film of Fig. A 
was in contact with the positive discharge wire. The film of B faced 
the end of the grounded wire. 
Plate VII.—Plates exposed as in VI, the discharge wire being a 
couple of mm. from the film of plate A. The fogging on both plates is 
due to the grounded end of the leakage wire. 
Plate VIII.— Plates exposed like VI, on the negative discharge wire. 
Plate IX.— Fig. A. Apparent refraction of electrical fogging by a red 
glass fiber laid on the film. A white glass fiber laid across the red one, 
and was not in contact with the film. Fig. B. An enlarged view of the 
focal line of A showing branching discharges from its ends. Fig. C. 
Fatigue effect in the glass fiber. 
Plate X.— Fig. A. Fatigue effects in the hard rubber holders. Fig. B. 
Arrest of positive discharge lines by pencil marks on the film. Fig. C. 
Arrest of negative outflow from a fogged area by pencil marks on the 
film. 
Issued February 18, 1910. 
