PIERCE. THE COOPER HEWITT MERCURY INTERRUPTER. 397 



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an intense luminescence throughout the bulb, while brilliant flashes are 

 thrown up from both electrodes all around the line of contact of the 

 mercury with the glass. It looks as if a great many of these little 

 fountains of fire occur simultaneously. The revolving mirror shows, 

 however, that their occurrence is usually successive, — each little flash 

 going through its series of oscillations and dying out before another 

 flash appears. It is thus not 

 difficult to make the expos- 

 ure so short that only one 

 fountain with its oscillations 

 appears on the plate. By 

 diaphragmiug the bulb and 

 adjusting the position of the 

 sensitive plate the pictures of 

 Plate I, Figures 1, 3, and 4, 

 were made to take in only 

 the illumination of the nearer 

 regions of the electrodes. The 

 mirror was turning in the 

 direction from the bottom of 

 the cut towards the top, and 

 the two vertical lines of im- 

 pressions in Figures 1, 3, and 

 4 are respectively the oscilla- 

 tions at the two electrodes. 

 The picture of Figure 3, 

 which is clearer, shows that 

 a bright point of light ap- 

 peared first on the electrode 

 to the left, and that this spot 

 persisted for a time sufficient 

 for the mirror to turn through 



a distance indicated by the length of the bright spot on the plate. 

 During this time the point of light widened a little and then died out. 

 Shortly after the extinction of the illumination at the left-hand electrode, 

 a bright spot appeared on the electrode at the right. After this dis- 

 appeared a second illumination occurred on the left, and so on for a 

 series of oscillations whose number depends on the self-inductance, 

 capacity, and resistance of the circuit. Figure 1 (Plate I) shows four 

 oscillations at a single electrode. Figures 2 and 5 of Plate I are the 



c 



Figure VII. 



