524 Dr. J. H. Vincent : Electrical Experiments 



the lamp 83. The pressure was 19'3 cm. of mercury. The 

 current is flowing through the lamp from the lower (time 

 signal) side to the upper side of the ribbon pattern. The 

 surface of the cathode is seen to fluctuate rapidly in bright- 

 ness as does the cathode in the necklace effect. These 

 alterations show no regular periodicity. The vertical light 

 and dark narrow bands represent irregular alterations in the 

 brightness of the arc. They are apparently unconnected 

 with the bright negative spots. 



The alteration in the appearance of an arc which changes 

 from a necklace to a ribbon is very marked even when 

 viewed directly. If looked at through red glass, the whole 

 ribbon is of uniform colour and brightness, but the waist of 

 a necklace is much brighter than the rest and contains more 

 orange light. 



In the case of the arc of constant length, the luminous 

 column generally moves slowly in the opposite direction to 

 the current due to evaporation from the anode. Occasionally 

 the ribbon effect continues for some time with great steadi- 

 ness, then a flicker occurs which is followed by another 

 period of steadiness. This flickering is met with in cases in 

 which the progressive motion of the column of light in the 

 direction opposite to that of the electric current is not notice- 

 able. It is, in fact, clue to an effect which is calculated to 

 lessen or even to prevent the motion of the illuminated 

 column of vapour. When such a lamp is examined with a 

 magnifying glass through a suitable colour screen, the 

 positive and negative ends appear quite different. Near the 

 positive end over a distance extending a millimetre or ^o 

 from the liquid mercury, the inner wall of the tube is seen 

 to be covered by mercury dew. This increases in amount 

 and the drops coalesce until a flicker occurs and the pheno- 

 mena are repeated. No deposition can be seen in the rest 

 of the space occupied by the vapour. 



Where a lamp is giving the ribbon effect under higher 

 voltages, it may go out from time to time and light up again 

 automatically. The green glow proceeds from the ends of 

 the vapour column while the main luminosity of the lamp is 

 absent. This was well shown by a quartz tube (10 cm., 

 4 3 mm., "46 mm,, 400 volts, 710 ohms) when the hydro- 

 static pressure was 29 cm. of mercury. The green glow 

 also appeared when the current was switched off. If the 

 current is turned on again while the glow is present, which 

 may last several seconds, the lamp relights. This is so even 

 when the direction of the current has been reversed. 



The ribbon effect may sometimes be obtained bv plunging 



