944 



NATURE 



[December 29. 1923 



those derived from pines and other coniferous trees. 

 Egypt produced no kinds of incense trees or shrubs. 

 The common incenses were pine resin, ladanum, and 

 myrrh, and all these were imported. It is difficult to 

 believe that the ceremonial use of incense arose in Egypt. 

 These arc a few of the (juestions raised liy a study of 

 the material relating to the origins of the ancient 

 civilisation of Egypt. An immense vista has been 

 opened out before our eyes by the discovcriei of the 

 last tiiirty years, and now, in Egypt better than in any 

 other country in the world, we can see man passing 



from the primitive hunter to the pastoral nomad, 

 the pastoral nomad to the agriculturist, and 

 on to the civilised life which l)egins with the art 

 writing. We can see in the Delta and in the 

 Nile Valley tribes becoming permanently settled 

 fixed abodes around primitive cult-centres, and tl 

 uniting with others into one community. We 

 trace the fusion of several communities i 

 States, and then, later, the uniting of Si 

 a supreme sovereign. What other country in the 

 world preserves such a record of its early history ? 



Rare Gas Discharge Lamps. 

 By J. W. Ryde, Research Laboratories of the General Electric G)mpany, Ltd., Wembley. 



A DISCHARGE of electricity through a gas at 

 atmospheric pressure generally takes the form 

 of a luminous spark which will pass only under 

 a potential gradient of several thousand volts per 

 centimetre. If, however, the pressure of the gas is 

 reduced, the appearance of the discharge changes. 

 First it spreads out into wavy streamers ; the streamers 

 then broaden until the discharge tube is filled with a 

 diffuse luminous glow extending from the positive 

 electrode to within a short distance of the cathode. 

 This glow is known as the positive column. The 

 cathode is now covered with a layer of bright luminosity 

 called the negative glow, and on close inspection is 

 seen to be not quite in contact with the electrode but 

 separated from it by a thin and sharply defined region, 

 known as the Crookes's dark space. Another less well- 

 defined dark region, the Faraday dark space, is between 

 the negative glow and the positive column. Further 

 reduction of the pressure results in a widening of both 

 dark spaces and the negative glow, the positive column 

 at the same time becoming correspondingly shorter. At 

 still lower pressures the Crookes's dark space increases 

 until finally it fills the whole tube and there is no 

 further luminosity of the gas. 



The potential necessary to start the discharge 

 depends on the pressure and nature of the gas, the 

 form and material of which the electrodes are made, 

 and also on the distance between them. In a given 

 gas with given electrodes the starting potential is large 

 at high pressures, but decreases, in an almost linear 

 relation, with decrease of pressure, reaching a minimum 

 at what is called the critical pressure, after which it 

 rises again very rapidly. The pressure at which the 

 minimum occurs depends on the form of the electrodes, 

 their distance apart and the nature of the gas, but 

 the minimum itself depends on the nature of the gas 

 and the material and form of the cathode employed. 

 It is about 200 volts for neon, 280 volts for hydrogen, 

 340 volts for air, and 420 volts for carbon dioxide. 

 Small trages of impurity affect these values to a great 

 extent ; thus, the addition of only 0-5 per cent of 

 pure hydrogen to neon reduces the minimum starting 

 potential by nearly 50 volts, 5-0 per cent reduces it 

 by about 60 volts, but if more than 5-0 per cent is added 

 the hydrogen ceases to act as an impurity and begins to 

 show its own characteristics, so that the starting voltage 

 rises again. On the other hand, small quantities of 

 other gases, such as oxygen, raise the starting potential 

 instead of lowering it. The material of which the 



NO. 2826, VOL. I 12] 



cathode is made has also some effect. The figures 

 given above refer to cathodes of ordinary metals such 

 as iron, nickel, or copper, etc., between which there are 

 only small difTerences, but with magnesium, barium, 

 or the alkali metals the starting voltage is < ly 



reduced ; in fact, with certain alloys of tli' ils, 



the glow discharge can be started in neon at a potential 

 so low as 90 volts and may be maintained at 58 volts. 

 When once the discharge has started, the potential 

 can be lowered somewhat before the H- ''"•"» -^-^ps. 



4001 



The curve connecting this " going out " voltage and 

 the pressure is roughly similar to the starting voltage 

 curve, but is displaced to a position somewhat below it 

 on the voltage ordinate. 



If the electrodes are only a short distance apart, so 

 that the positive column is absent, the relation betwet-r 

 the current flowing and the potential applied across tl 

 electrodes is that shown in Fig. i. The states repre- 

 sented by the lower and upper parts of the curve are 

 unstable, and can be observed only if there is in series 

 with the discharge an appropriate resistance, by mean.^ 

 of which the current may be varied. With this arrange- 

 ment, when the current is only of the order of a micro- 

 ampere, a faint glow is seen some distance from the 

 cathode. The potential between the electrodes falls 

 rapidly as the current is increased, and at the same time 

 the glow becomes brighter and assumes the form of a 

 sharply defined patch moving nearer and nearer to the 

 cathode as the current rises. The voltage now remains 

 sensibly constant, being that known as the normal 

 cathode fall, which is the lowest potential at which 



I 



