942 



SCIENCE 



[N. S. Vol. XLII. No. 1096 



COLOR EFFECTS OF POSITIYE AND OF CATHODE RATS 

 IN RESIDUAL AIR, HYDROGEN, HELIUM, ETC. 



As is well known positive rays have their 

 origin in front of the cathode, and under the 

 action of the electric force fall toward it. If 

 the cathode is perforated the rays stream 

 through and constitute the "kanal strahlen" 

 of Goldstein. Tubes built to exhibit this phe- 

 nomenon form a part of the regular equip- 

 ment of nearly all collections of apparatus in- 

 tended to exhibit the phenomena of electric 

 discharge through gases. 



Most beautiful and striking color effects 

 may be had by using hollow cathodes^ in 

 specially designed tubes containing each a 

 trace of some inert gas such as helium, argon 

 or neon. The color effect is striking because 

 the cathode beam is of one color, while the 

 positive ray beam in the same gas is of an en- 

 tirely different color. The general design of 

 the tubes that Dr. Jakob Kunz and the writer 



have found best suited is shown in the accom- 

 panying figure. The discharge tube is dumb- 

 bell shaped. It is made of two 2 liter Florence 

 flasks, M and N. The hollow cylindrical 

 cathode is mounted in the neck, while the 

 anode A is placed in one of the bulbs. The 

 cathode terminal 0, the nipple p for exhaust- 

 ing, and the charcoal bulb B are all attached 

 to one vertical tube as shown. 



The process of filling the discharge tube, 

 1 J. J. Thomson, "Eayg of Positive Electricity," 

 p. 6, 1913. 



sealing it off from the pump, and its subse- 

 quent use is as follows : After the tube is con- 

 structed, and the charcoal bulb B attached, 

 the exhaust nipple is put in communication 

 with a pump, and also to some source of the 

 gas to be used. During the early part of the 

 exhaustion it is well to gently heat the bulb B. 

 Continue the pumping until the tube on 

 sparking shows a tendency of becoming hard. 

 As this stage is approached cathode rays will 

 appear as a compact beam in the bulb N, 

 while a beam of positive rays will traverse the 

 bulb M. Now admit a small quantity of the 

 desired gas, say, helitmi. The chances are that 

 too much gas will enter the discharge tube and 

 thus destroy the definition of the two beams. 

 To restore it pumping should be continued 

 and at the same time the bulb B should be 

 carefully submerged in liquid air. Care must 

 be exercised not to reduce the content of 

 helium by too long continued pumping. The 

 cooled charcoal wiU absorb the traces of air 

 leaving the tube MN relatively richer and 

 richer in helium — since heliiun, an inert gas, 

 is but slightly absorbed by the cooled charcoal. 

 The cathode beam in N as well as the positive 

 ray beam in M will each increase in bright- 

 ness and definition, reaching a maximum, 

 after which, as the process continues, they will 

 begin to fade. At the stage when the beams 

 are judged brightest the exhaust nipple p is 

 sealed off from the pump. The tube is now in 

 its finished state. Removing the liquid air, 

 the charcoal gives up its absorbed gas and the 

 beams weaken and become diffused. For sub- 

 sequent use it is only necessary to submerge 

 B in liquid air while the discharge from an 

 induction coil is passing. The beams in M 

 and N will increase in brightness and defini- 

 tion as the absorption of the active gases pro- 

 ceeds, thus giving ample time for the observa- 

 tion of the changes going on within the tube. 

 The most interesting phenomenon is the 

 color of the two beams. The cathode beam in 

 helium is a greenish gray color, while the posi- 

 tive ray beam in the same gas is a beautiful 

 red. There is no mistaking the colors. In- 

 deed the red due to the positive ions is so per- 

 sistent that it appears at the very origin of 

 these rays — at the edge of the Crookes dark 



