238 



Seven series of photographs were then taken. 

 A — Is a visible spark discharge. 



B — Is a Brush discharge (a violent stream extended about 0.8 cm. 

 from the positive point. The negative point showed only a 

 bright speck). 

 C — The glow or silent discharge. (Nothing was visible between the 

 points in the darkened tube. Each point showed a bright 

 speck.) 

 D — Spark discharge representing the highest speed of the machine 



and highest potential between the points. 

 E — Spark discharge, representing the lowest speed of the machine at 

 which a visible spark passed between the points. A lower 

 speed would have caused the spark to change to brush. 

 F — Silent discharge, same as C. 



G — Silent discharge, same as C. Deflected by a stream of air issu- 

 ing from below the points. 

 The different series in decreasing order of their potential as repre- 

 sented by the relative speed of the machine are D, A. E. B (C, F or G). 

 Series A, B and C were taken with magnet and pole pieces as represented 

 in Fig. 6. The magnetic field strength was about 1,000 lines per sq. cm. 

 in the region of the points. The points were 15.7 mm. apart. Series 

 D, E and F were taken with the magnets as shown in Fig. 8. The mag- 

 netic field strength in the neighborhood of the points was about 1.500 lines 

 per sq. cm. 



The six numbers of each series, A. B, C, D. E and F, were taken in 

 succession as rapidly as possible, it requiring 20 or 30 minutes to complete 

 the series. In the photograph the longer stream is the one from the posi- 

 tive terminal and the shorter one the stream from the negative electrode. 

 If the positive stream is from right to left it is designated as "first direc- 

 tion," if from left to right, as "second direction." Nos. 1, 2 and 3 then 

 show current in the "first direction," while Nos. 4, 5 and 6 show current 

 in the "second direction." If the magnets were excited so that the direc- 

 tion of the lines of force were from the front to the back of the photograph 

 (i. e., after correcting for the reversal in direction caused by printing 

 from the plate), the direction of magnetism is designated as "first direc- 

 tion," and those with the lines of force from back to front of the page are 

 designated as magnetized in the "second direction." 



