December 29, 1923J 



NATURE 



945 



the unsupported discharge can be maintained. With 

 a further increase of current, the glow spreads laterally 

 over the cathode, its intrinsic brilliancy and the current 

 density remaining practically constant. A value of the 

 current is eventually reached at which the cathode 

 becomes completely covered with a layer of glow 

 separated from it by the Crookes's dark space, which is 

 only a fraction of the width of the layer of glow itself. 

 Any further increase in current brings us to the part c 

 of the curve ; the voltage now increases with increase 

 of current and the glow becomes brighter and the dark 

 space narrower. The curve eventually becomes very 

 steep, following the path cc' , and currents df the order 

 of several amperes per square centimetre of cathode 

 surface can be passed. At some part of the curve cc 

 the cathode begins to heat up considerably, and if 

 composed of a refractory metal such as tungsten, will 

 become white-hot; thermionic emission then takes place 

 md the glow discharge passes over to the arc discharge. 

 As soon as this begins the potential difference between 

 the electrodes begins to decrease as the current rises, 

 and may finally drop to only a few volts. 



Gas discharge lamps may be divided into three 

 (lasses, namely : (i) Lamps in which practically all the 

 light is emitted by the positive column, (2) negative 

 glow lamps in which the positive column is absent, and 

 (3) lamps in which the light is emitted not from the gas 

 but from the cathode itself^ which is rendered incan- 

 descent by the discharge. 



Lamps of the Moore type belong to the first class. 

 They consist of a long glass tube filled with gas to a 

 pressure of a few millimetres and having an electrode 

 at each end. On account of the distance between the 

 electrodes, the operating potential usually amounts to 

 several thousand volts and is inconveniently high. 

 Recently, short tubes of this type containing neon, and 

 having electrodes made of alkali metal alloys, have 

 been developed which will run on 220 volt A.C. supply, 

 but require a special device giving a higher voltage for 

 starting. These lamps are very efficient; with some of 

 the tubes an efficiency of 0-65 watts per candle is 

 obtainable. The colour of the light, however, is a 

 brilliant orange red, which for many purposes is objec- 

 tionable. 



The second class of lamp was developed to run 

 directly on ordinary lighting circuits. The electrodes 

 are placed a few millimetres apart in a small bulb, 

 the distance between them being such that the positive 

 column is absent ; this is to enable them to start and 

 run at ordinary supply voltages. The gas used for 

 filling the lamps is neon with about 20 per cent, of 

 htliuni. which is separated with it during the process 

 ol (xti.i tion from the air. The advantages of neon 

 are tlincluld'. ' ' ' ulace, most of the energy 

 radiated linin t lies in thevisilile spectrum; 



secondly, tin- -m iili;il i; lower tli;m in other 



gases; and 1 M' ' '-■;;■ o' ?'' h l-i ^ ii'ittrd. hcing 



;i ' !liiinm,it ion 



V .' ' ,^ • '"}' using 



hydro.Licn to rcdiKc th(^ >t.iitinL' voltage as explained 

 above. ;ind ii I'lliii: i'm Imih to about the critical 



pressure'! - m,i|.,i 1, mips having 



iron clei t tin,. , ,i, >,,,,. n , ,1. . i. - ,0 -<■ w ill >hir1 at about 

 150 volts. 



The whole ol till- liulit Ironi these lamps comes from 



NO. 2820, VOL. I 12] 



the negative glow,' which appears as a layer of bright 

 orange luminosity about two millimetres thick com- 

 pletely covering the cathode, whatever its size or shape. 

 The Crookes's dark space at the pressure used is only a 

 fraction of a millimetre thick and is not easily seen. 

 In consequence, the luminosity takes the form of the 

 cathode, so that if this electrode is formed out of a sheet 

 of metal in the shape of a letter, numeral, or similar sign, 

 it will appear brilliantly illuminated when the discharge 

 is passing. It is in this form that the tube is used for 

 advertising purposes. In cases where the lamp is 

 required for dim illumination, the cathode is made of a 

 spiral of wire in the shape of a beehive, which ensures an 

 approximately even distribution of light in all directions. 

 The other electrode is either an iron wire hidden behind 

 the letter, or a plate inside the spiral. On direct current 

 only one electrode becomes illuminated, but on alter- 

 nating current the small electrode also glows during the 

 half cycle when it becomes the cathode. 



On account of the rigidity of the mechanical con- 

 struction, the lamps are robust and their life is limited 

 only by their progressive blackening. This blackening 

 is due to particles of the cathode which are shot off 

 during the passage of the discharge and collect in the 

 form of a film on the bulb. The rate at which the 

 blackening takes place depends on the current passing 

 through the lamp, and also varies very greatly with the 

 particular metal used for the electrodes and the nature 

 of the gas. The addition of impurities which lower the 

 starting potential of the gas also considerably reduces 

 thfe blackening. Thus, with iron electrodes and pure 

 neon, the useful life of the lamp would only be about 

 80 hours, while with an addition of a small percentage 

 of hydrogen the life becomes of the order of 1000 hours 

 or more. A series resistance placed in the caps of the 

 lamps makes them suitable for various voltages, and at 

 the same time reduces the current to a value leading 

 to a reasonable life. 



These lamps, in common with all forms of Geissler 

 discharge tubes, possess many properties which are 

 valuable for purposes other than that of lighting. For 

 example, when running on the part ah of the curve of 

 Fig. I they can act as a negative resistance and can be 

 used to generate oscillations. They are also sensitive 

 detectors of current; the luminosity produttil by a 

 current of one microampere is easily seen if the lamp is 

 shielded from direct daylight. But these other develop- 

 ments cannot be treated here. 



In the third class of lamp, the electrodes are usually 

 tungsten spheres about one millimetre in diameter and 

 placed one millimetre apart. The bulb is filled with 

 neon to about 50 mm. pressure. When first switched 

 on, an intense glow discharge takes place which heats 

 the cathode wliite-hot ; a thermionic emission then 

 ensues and tlie ])otential ai ross the lamp drops to about 

 25 \dlts. the rnrrent being i*o to 1*25 amperes, the 

 remaining; \olis bein- dropped in the series resistance. 

 The lain]) al llii^ tinal stage operates in llii r. . ion e 

 ol the ruin lit \ oltage curve shown in l-i::. 1, The 

 uhite hot iniiL'^ten ball acts as a ver\ iiitiiise point 

 siiiiii. .1 li lit suitable for projection pinjioses. I'he 

 I'ni! ip is a variant of this form, in whii h the 



clis( I; initialed In- means of a tungsten spiral 



heato M i(ll\ . tin heating current being cut off 

 when the lamp has started. 



