290 PROCEEDINGS OF THE AMERICAN ACADEMY. 



by a split solenoid of fine insulated copper wire wound on a paper 

 tube. This device completely shields the cathode beam from electro- 

 static disturbances, but allows the magnetic forces to pass undiminished. 



When the Braun tube is used for alternating currents of commercial 

 frequencies, or for phenomena taking place in a few hundredths of a 

 second, the velocity of the luminous spot over the screen is small 

 enough to give a visible streak of light even if the phenomenon lasts 

 for only one cycle. 



When high frequency currents are used, however, the velocity of de- 

 flection of the cathode beam is so great that it is necessary that the 

 luminous spot travel over the same path thousands of times a 

 second in order that even a visible effect may be observed, and many 

 more times if a photographic record is to be taken with a reasonable 

 exposure. 



It is then apparent that, if a sequence of instantaneous phenomena 

 is to be delineated by causing the cathode beam to trace out a pattern 

 on the fluorescent screen of the Braun tube, the c)'cle of events must 

 repeat many hundreds of thousands of times a second, and with such 

 regularity that the spot of light will, each cycle, travel over exactly 

 the same path. 



When using the Braun tube for studying electrical oscillation, this is 

 usually more or less of a difficult condition to attain on account of the 

 uncertain and variable action of most forms of discharge gaps. The 

 gap and system under consideration is, however, so regular and con- 

 stant that it is not at all difficult to obtain photographs of patterns 

 representing cyclic changes, many of which recur three million times a 

 second with practically no variation. 



Plates 1-7 are a few of over fifteen dozen photographs taken repre- 

 senting various conditions, and give unmistakable evidence of the 

 remarkable regularity and continuity of the oscillations of the Cu-Al 

 gap. The photographs were taken on Seed's Gilt Edge No. 27 plates, 

 and exposures of from 3 to 30 seconds were made. The figures are 

 practically natural size. 



(2) The Primary Wave. 



The observations for determining the shape and phase, with respect 

 to the secondary oscillation, of the primary current rush were obtained 

 by leading the primary and secondary currents each through a 4-turn 

 deflecting coil about the Braun tube, the two coils making an angle of 

 90° with each other. The arrangement is diagrammatically shown in 

 Figure 13, where the letters have the before mentioned significance. 



