THE MERCURY ARC 



99 



current was 6.7 amperes and the pressure of the vapor 

 was 1.33 mm. This is shown in Fig. 29. The total 

 potential difference was 22.5 volts. It is of interest to 

 note that there is a point near the anode ' where the 

 electric force is in the opposite direction to that in which 

 the current is flowing. This is no doubt due to an accu- 



DISTANCE FROM ANODE 

 FlG. 29. 



mulation, in this neighborhood, of ions whose electrostatic 

 effect is sufficient to reverse the original direction of the 

 electric force. This accumulation is due to the momen- 

 tum of the ions which carries them for a short distance 

 against the electric force. 1 



The vapor of the mercury arc remains highly conducting 

 and quite luminous even after it has passed from the arc 

 into a condensing chamber. Stark 2 found that if this vapor 

 is passed through a magnetic field, an E.M.F. is induced, 

 which is perpendicular to the field and to the direction of 

 the motion of the vapor. The magnitude of this E.M.F. 

 indicates a velocity of the vapor of approximately 280 

 meters per second. 



1 Phil. Mag., (6), 18, 442; 1909. 



2 Phys. ZS., 4, 440; 1903. 



