Specific Velocities of Ions in Discharge from Points. 79 



Dr. D. K. Morris, at Mason College, Birmingham, pre- 

 liminary experiments having been performed in London at 

 University College in the laboratories of Profs. Ramsay and 

 Callendar. I have pleasure in expressing my thanks to these 

 gentlemen, as well as to Mr. A. W. Porter, of University 

 College, London, for friendly interest shown in my work. 



IV. On tlie Specific Velocities of Ions in the Discharge from 

 Points. by A. P. Chattock, Professor of Physics, 

 University College, Bristol; and Winifked E*. Walker, 

 B.Sc, and E. H. DlXON, B.Sc, Associates of University 

 College, Bristol*. 



IN a communication to the Philosophical Magazine for 

 November 1899, it was shown by one of us that under 

 suitable conditions the pressure of the electric wind furnishes 

 a means of determining the specific velocities of the ions con- 

 cerned in its production ; and results obtained in the case of 

 air were given in illustration of the method. 



Subsequent experiments on other gases, while leading to 

 values which were roughly in accord with the velocities of 

 ions obtained by #-rays, have remained unpublished on 

 account of the behaviour of hydrogen. The results for the 

 negative ions in this gas were found to vary between the very 

 wide limits 5*7 and b'3, and in later work 4*7 and lOO centi- 

 metres per second in a field of one volt per centimetre. 



It is true that, owing to the smallness of the wind-pressures 

 for hydrogen, exceptionally large errors w^ere to be expected 

 in the readings of the pressure-gauge, and hence in the 

 deduced velocities ; but the magnitude of the observed varia- 

 tions was much too great to be accounted for in this way, 

 and it seemed possible that these might be due to real changes 

 in the specific velocities themselves. 



Analysis of the results now obtained, without being abso- 

 lutely conclusive, leaves little room for doubt that this view- 

 is correct. 



Specific ionic velocities have been measured in the five 

 substances, Hydrogen, Carbon Dioxide, Air, Oxygen, and 

 Turpentine according to the formula 



Y = zlc P , 



where c is the current from the point, and p the increase in 

 the total wind-pressure corresponding with a shift of the dis- 

 charging point through z along the axis of the discharge- 

 tube (/.' c). 



* Communicated bj the Authors. 



