20 Dr. E. N. da C. Andrade on the Electrical 



§4. Calculation of the number of Free Electrons 

 in the Metallic Vapour. 



If we assume a given velocity per unit gradient for the 

 negative carriers of the flame we can calculate from the 

 current they convey their number, since we know the charge 

 per electron. We know also the number of metal atoms in 

 a given volume of the Sr vapour, since we know from expe- 

 riment the rate of vaporization for a given brilliancy *. 

 Hence we can find the number of free electrons per Sr atom, 

 or assuming that these are produced by the loss of an 

 electron from a Sr atom, the fraction of the time during 

 which the atom is positively charged. 



To get the velocity of the electron we must know the 

 potential gradient in the flame. The distribution of potential 

 was found by measuring the difference of potential between 

 the positive electrode and a parallel fine platinum wire 

 inserted in the flame, the position of which could be regulated 

 by a screw. The gradient was found to be very great by 

 the negative electrode and gradually diminished towards the 

 other electrode. If the electrons had the same velocity in all 

 parts of the flame, the potential gradient ought to be uniform. 

 But the cold negative electrode cools the flame in its neigh- 

 bourhood strongly, so that probably the electrons only possess 



the velocity per — : - which they normally have, and which 



Wilson f has measured, at some distance from it. i.e., near 



the positive electrode. We therefore take as the potential 



gradient in question the gradient near the positive electrode, 



and calculate the velocity of the electrons from it. 



We take a case where the brightness of the streak, 



measured as described later, was 1*88 arbitrary units. In 



this case the potential fall per cm. near the positive electrode 



. volt 



was 6 . 



cm. 



Weight of SrO vaporized =0*15 mg. per hour. 



=4x 10" s gm. per sec. 

 Height of the electrode =0*8 cm. 

 Velocity of flame =450 



BOO* 

 .*. Weight of metallic Sr in a portion of the streak 



* See § 7. 



t H. A. Wilson, Phil, Trans. A. excii. p. 490 (1899). 



