30 Dr. E. N. da C. Andrade on the Elect 



Deflexion o£ the weaker 9treak. 



Pressure. 



Streak angle. 

 a'. 



Pyrometer. 



a — a'. 



a' -9. 



44 



17° 



About 1-72. But i 



15° 



4° 



39 



16° 



for such small 



ISO 



4° 



32 



16° 



luminosities 



17° 



3° 



2-35 



17° 



the instrument 



16° 



4° 



205 



17° 



1 is unreliable. 



14° 





1-3 



17° 





19° 





10 



17°-5 





15°o 



35° 



To find the true velocity of migration per - we must 

 J o i cm 



know the potential gradient in the flame, and the flame 

 velocities. The latter were measured by bringing small 

 carbon particles into the flame, and observing it in a rotating 

 mirror*. The flame velocity diminishes linearly with in- 

 creasing presure ; the relation is exhibited in fig. 5 (a). 

 The fall of potential in the flame was measured by bringing 

 two fine parallel platinum wires into the flame, and measuring 

 the potential difference between them with a calibrated 

 aluminium leaf electrometer ; two different distances of the 

 fine wires were employed. The potential gradient in the 

 flame increases with the pressure when the potential of 

 the insulated electrode is kept constant : this is to be expected, 

 since the conductivity of tin 1 flame decreases very rapidly 

 witli increasing pressure. The variation el' the gradient in 

 a flame of fixed size with the pressure, the potential difference 

 between the electrodes being constant, is shown in fig, 5 (A). 

 From the angle of deviation of the streak and the curves 

 and (/>) of fig. 5, the dependence of the velocity of migration of 

 the positive carriers or the luminous streak upon the pressure 

 can be calculated. The experimental numbers are shown in 

 the following table ( 1 1.) ; they refer to the stronger streak. 



The product r x pressure, exhibited in the last column o\' 

 the table, is roughly constant ; the variations arc no! larger 

 than arc to be expect d from unavoidable errors in the 

 different measurements. Thus from 1 to 1 atmospheres the 

 velocity of migration is inversely proportional to the pressure: 

 the experimental curve connecting these quantities is shown 

 in fig, . r » (>). 



+ For detail of the method see 

 (4) xwvii. p, 380 ( 1912). 



I \ , da C. Andrade. Ann. der Phmk 



