300 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



through the gap, and hence no current deflection, but the potential of 

 the condenser increases to a certain value determined by the breaking- 

 down potential of the gap. This charging is represented in the 

 photograph by a movement of the luminous spot from A, its undeflected 

 position, to B, the potential at which the gap breaks down, which in 

 magnitude, in this case, is equal to about 450 volts. The instant that 



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Figure 17. 



the gap breaks down the potential drops, and the current through the 

 gap increases giving the curve, from B to C, roughly resembling a hyper- 

 bola. As the current decreases, represented by the line CA in the 

 photograph, the potential remains practically zero, showing that after 

 the gap has been once ionized the resistance remains low until the 

 current becomes zero. The current does not increase in the negative 

 direction for reasons previously given. During the passage of the 

 primary current the capacity, Ci, has been completely discharged and 

 charged in the reverse direction so that, as soon as the current stops, 

 the full potential of the reversed condenser is brought to bear across 

 the gap. This is represented by the point Z>. Current is now, however, 

 flowing into Ci at a constant rate, causing the fluorescent spot to move 

 along the voltage axis from D to B to begin a new cycle. 



The same kind of diagram is shown in cuts b and c of Plate 3 for 

 shorter gaps. It is evident, as would be expected, that as the gap is 

 shortened, the B-I curve closes in toward the two axes. 



It is also seen that with a very short gap the current increase is 

 oscillatory. These oscillations are very rapid, having a wave length of 

 the magnitude of four or five meters, and on account of the extremely 

 high fi-equency cause the large potential oscillations shown by the 

 blurred space in cut d of Plate 3. 



The reason for these su])erposed higher frequency oscillations is easy 

 to understand if we consider again the rapid drop in resistance repre- 



