20 Trans. Acad. Sci. of St. Louis. 



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 as in 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. 



