72 Trans. Acad. Set. of St. Louis. 



EXPLANATION OF THE PLATES. 



Plate XVIII. — Fig. A. Electrical discharge from pin-head terminal. 

 Negative outflow in negative line. Fig. B. Negative inflow to pin-head 

 in negative line. 



Plate XIX. — Fig. A. Positive line. Positive outflow. (Negative in- 

 flow). Fig. B. Positive line. Positive inflow. (Negative outflow). 

 These four figures are emphasized by condenser effects. 



Plate XX. — Fig. A. One minute spark. Negative line. Fig. B. 

 Slightly longer spark. Fig. C. Two sparks. The sparks were in a small 

 gap at the machine. 



Plate XXI. — Fig. A. One spark. Increased length. Fig. B. Five or 

 six sparks and then increase of speed of machine and disruptive spark. 

 Fig. C. Dispersion of spark discharge into ionized air around grounded 

 terminal. Shadow of pin-head terminal. 



Plate XXII. — Fig. A. The same. Fig. B. The same. Two discharges 

 in quick succession. Fig. C. The same. One spark. Fig. D. Photo- 

 graphic reversal of last Fig. 



Plate XXIII. — Fig. A. Shadow effects at both pin-heads. Explosive 

 discharge channel very short. Fig. B. Effects like preceding but in pos- 

 itive line. Fig. C. Pin-head terminals connected to + and — terminals 

 of machine. Small gap at machine. Spark about to pass between pin- 

 heads. Compare Fig. A, Plate XXI. 



Plate XXIV. — Fig. A. Camera photograph of brush discharges. In- 

 sulated copper plate between terminals. Spark will not pass. Fig. B. 

 Same as last with drill-hole in copper plate. Eighty spark discharges. 

 Pin-hole photograph. Fig. C. Camera photograph of canal ray. Same 

 as last with no Leyden jars in machine. Brush discharges. 



Plate XXV. — Fig. A. Shadow by glass tube in positive column. 

 Camera photograph. Fig. B. Brush discharge in which mica windmill 

 shows feeble positive "electrical wind" or none. Camera photo. Fig. C. 

 Same where marked negative wind is shown. Fig. D. Brush discharge 

 at a 1 Fig. 2 showing Faraday dark space and positive column. Camera 

 photograph. This spark gap was 3.2 cm. in length. 



Issued June 2, 1910. 



