Mobility of the Negative Ion. 443 



In a previous note the author * has shown that if two 

 parallel plates are immersed in an ionized gas, a small 

 potential difference established between the plates produces 

 a wind-pressure large enough to be measured by a special 

 gauge f , which will be described later on. When a surface 

 ionization of one sign close to one of the plates is produced, 

 the density of electrification, as well as the strength of the 

 field, is uniform throughout the volume between the plates 

 (if the disturbing effect of the ions is neglected), and in this 

 case Ohattock's formula maybe deduced in a very elementary 

 way. Let P be the total surface electrification on the plate, 

 X the strength of the field, iv the velocity of the ions, c the 

 total ionization current, P the corresponding wind-pressure, 

 and d the distance between the plates. Then Yw is the 

 quantity of electricity streaming through a plane parallel to 

 the plates in unit time and c = Pw. When the ions move 

 with a uniform velocity their drag on the gas is equal to the 

 product of the total electrification between the plates and 

 the strength of the field J. Hence P = 6?yoX. 



These two equations give 



x~ d }>' ^ 



where ^ is the mobility of an ion. In the case when the 



velocity id of an ion is not proportional to the electric force X, 



^ is what is called the abnormal mobility of an ion. 



The experiments may be carried out at any pressure, the 

 potential difference between the plates varying over a very 

 wide range, viz. from zero up to that required for a spark- 

 discharge between parallel-plate electrodes, and, as will be 

 shown later, by this method ionic velocities may be measured 



of the order of from 10 to 10 7 — ! . 



sec. 



The apparatus used is shown in figs. 1, 2, and 3. Two 



brass plates A and B (fig. 1) are mounted on a wooden 



base ST. The circular plate A, which consists of two 



connected metal sheets 8 cm. in diam., is supported by a 



rod / sliding in a brass collar e and is insulated by an 



* Comptes Bendus, clviii. p. 565 (1914). 



f I wish to thank Prof. Debierne for suggesting to me the idea of 

 this gauge. 



X This is experimentally proved by the fact, that using the above 

 formula Chattock deduced for the mobility of an ion the same value as 

 given by other methods. 



2H2 



