Mobility of the Positive Ion in Fl 



ames. 



781 



The potential curves thus obtained for the salted flame 

 exhibit three marked characteristics : (1) a small anode fall ; 

 (2) a region of small constant electric force (this is the 

 quantity required for determination of mobility) ; and (3) a 

 very sudden cathode drop. As the temperature is reduced, 

 the following modifications occur : — (1) the anode fall in- 

 creases slightly; (2) the region of constant electric force 

 contracts, and the force itself increases ; and (3) the cathode 

 fall becomes less abrupt. Finally, when a temperature of 

 1150° is reached, the second and third branches of the curve 

 merge into one straight line, but the anode fall persists* On 

 account of the presence of the suit bead, no readings could be 

 taken within less than 4 mm. of the anode. Thus it seems 

 as if below 1150° one can regard the fall of potential in a hot 

 gas as uniform. Wilson * made this assumption in working 

 with hot air at 1270°, in which case it is not quite true. Spe- 

 cimen results at four temperatures are plotted in fig. 3, in 



Fig. 3. 

 Fall of Potential in Salted Flame. 



10 15 20 25 



£>/Sr/l/VC£ f/fOM /lNOD£ /a/ mm . 



which ordinates represent potential and abscissa? distances from 

 the anode. The results obtained at the other temperatures are 

 intermediate in their characteristics, and are omitted to avoid 

 crowding the curves. For convenience in plotting, all the 

 curves have been reduced to a common potential-difference 

 between the electrodes, viz. 100 volts, though it must be 

 understood that the potential actually applied in each case 

 was that just required to reverse the motion of the positive 

 ions. The temperatures stated are the mean values for that 

 part of the flame in which the minimum electric intensity 



* Wilson, Phil. Trans. A. 1899. 

 Phil. Mag. S. 6. Vol. 22. No. 131. Nov. 1911. 3 F 



