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. 



