Behaviour of Metallic Vapours in Fla 



mes. 



19 



platinum strip, as before, since no anomalous effect was 

 found at the positive electrode. On displacing the streak of 

 vapour now from the positive towards the negative electrode 

 the current increases, but on contact of the vapour with the 

 neo-ative electrode there is no abnormal effect, and if the 

 luminous streak be still further displaced in the same 

 direction (i. <?., so that only part of the streak lies between 

 the electrodes) the current decreases, as we should expect. 

 We thus have a perfectly definite maximum corresponding to 

 the streak just touching the cooled negative electrode : the 

 type of curve obtained is shown in fig. 2. It will be seen 



Fig. 2. 



50 



25 



A 



t f 



Position of the luminous streak — *■ 



that the maximum is perfectly definite, and the glowing- 

 positive electrode exercises no abnormal influence. 



The form of the curve and the maximum when the streak 

 is in contact with the negative electrode is explained by the 

 fact that the positive carriers have a very small velocity as 

 compared with the negative. Consequently, if the streak is 

 not in contact with the negative electrode, nearly the whole 

 fall of potential takes place between the streak and the 

 negative electrode. When the streak touches the negative 

 electrode the small velocity of the positive carriers has little 

 effect, as they have onlv a short distance to travel. 



C2 



