788 



SCIENCE 



[N. S. Vol. XLIII. No. 1118 



three-quart battery jar. A hole bored through 

 the bottom receives the evacuating tube, the 

 junction being made airtight with ordinary- 

 sealing wax. The lip of the jar is ground flat 

 to receive the plate glass lid. The junction 

 here is made by means of the frequently used 

 half-and-half wax, beeswax and resin. This 

 wax because of its low melting point admits 

 of easy removal of the glass plate. The elec- 

 trodes are concentric cylinders and may well 

 be made of sheet aluminum — one electrode to 

 fit snugly the inner wall of the jar, and the 

 other mounted on a cylinder of glass tubing 

 about IJ inches in diameter, which in turn is 

 supported accurately concentric by sealing 

 wax from the bottom of the jar. Outside con- 

 nections to the electrodes are made by fine 

 bare copper wire run out through the waxed 

 joints. The assembled discharge vessel is 

 shown at a in Fig. 1. 



"tofump 



The vessel may be exhausted by a Gaede 

 mercury or a Gaede piston pump and, if de- 

 sired, the vacuum carried farther by the use 

 of charcoal and liquid air, though the latter is 

 not necessary. The potential employed by 

 the writer to produce the discharge was fur- 

 nished by a cabinet of high potential storage 

 cells of 1,000 volts. 



Two methods of operating were employed. 

 In the first an adjustable water resistance is 

 connected in series with the cells and dis- 

 charge vessel as shown at i in Fig. 1. When 

 the vacuum is right a beautiful discharge will 



make its appearance as patches of light on the 

 electrodes. These patches of light, when there 

 is considerable resistance in the circuit and 

 the vacuum is not very high, will be opposite 

 each other and the discharge, as a whole, will 

 wander about, sometimes swinging entirely 

 around, or at times travelling to the edges of 

 the electrodes, only to break away and move 

 to some other point. The movement of the 

 cathode glow (which is the smaller and hence 

 the brighter) is similar to that of the cathode 

 star over the surface of mercury in a mercury 

 vapor lamp. These areas grow as the vacuum 

 improves when ultimately the entire surface 

 of each electrode is covered. Or, with the 

 vacuum kept constant, the areas may be made 

 to increase in size by cutting out resistance. 

 Hence by improving the vacuum and at the 

 same time cutting out resistance the dis- 

 charge, if the inner cylinder is made cathode, 

 grows rapidly into a brilliant bull's-eye. The 

 appearance is very realistic, for if now resist- 

 ance is cut in, the dark space around the 

 cathode (as is evident after a moment's re- 

 flection) grows smaller, and vice versa. Its 



EiG. 2. 



outline is exceedingly sharp and perfectly 

 steady, and yet, though the discharge appears 

 very brilliant, the current required may not 

 exceed 20 milliamperes. 



This form of discharge vessel offers an in- 

 teresting method for the study of the stria- 

 tions and their relative spacing with reference 

 to the impressed discharge potentials. These 

 effects are best shown when the vacuum is not 

 too high and the discharge potential is ad- 

 justed to give a patch on the cathode, which 



