on the Direct Current Corona. 283 



symmetrical with respect to the time axis. The same 

 phenomenon can also be seen in a spherical glass bulb of 

 about 8 cm. diameter, one half of whose inner surface is 

 silvered, and connected with an electrode. The other 

 electrode is a copper ball of 3 mm. diameter in the center 

 of the glass bulb. If the tube is filled with nitrogen at 

 about one half of an atmosphere and subjected to a poten- 

 tial difference of 4500 volts, then the central ball when 

 positive is covered with a very thin uniform layer of 

 light, If, however, a small spark gap is introduced in the 

 circuit, more than half of the bulb is filled with most 

 beautiful streamers of purple light. It is indeed one of 

 the most beautiful phenomena of electric discharge. If 

 we use hydrogen instead of nitrogen, at somewhat lower 

 pressure, the streamers appear in the same way as in 

 nitrogen, but in addition, the negative silver mirror 

 shows a most beautiful display of bright scintillations. 

 Fig. 2a represents a photographic picture of the 

 streamers in nitrogen. According to the length of the 

 spark gap the streamers may be well separated from 

 each other, or flow together, filling the tube with a more 

 or less uniform light. 



In order to find the wave form of the current through 

 the tube, different methods suggest themselves. The 

 oscillograph and the vibration galvanometer can not be 

 used, because the current is too weak and the inertia ele- 

 ment too large. Three methods, however, have been 

 used which gave accordant results ; the telephone, revolv- 

 ing lenses and a photographic plate, and a cathode ray 

 tube with a hot lime cathode. The last method can be 

 made very sensitive and gives satisfactory results. 



In the first experiments a telephone receiver connected 

 in parallel with a resistance was used in place of a gal- 

 vanometer. The passage of the faintest spark can be 

 detected. When the voltage is high enough to produce 

 corona, as each consecutive spark passes a sharp click 

 will be heard in the telephone and a flash of glow will 

 appear on the wire in the tube. If the sparks pass in 

 very rapid succession the glow will appear to be prac- 

 tically continuous. The discharge between the spheres 

 has the character of a spark rather than of an arc. It is 

 intermittent, forming a white line in the gas between 

 the electrodes. 



