Mobility of the Negative Ion. 455 



the ion with a larger velocity is small compared with the 

 distance between the plates and may be neglected. 



3. It was supposed at first that the electrie field between 

 the plates was uniform. This, however, is not the case, the 

 electrification between the plates being large enough in these 

 experiments to disturb considerably the uniformity of the 

 field. Unfortunately, this source of error cannot be elimin- 

 ated or even diminished, since the ionization-current must 

 be considerable in order to produce a perceptible wind-effect. 

 It is useless to increase the sensitiveness of the gauge 

 beyond certain limits, as the electric wind produced must be 

 large compared with the convection current due to the heated 

 strip. There is, however, sufficient evidence to show that 

 the results are but little affected by this source of error. In 

 almost all the experiments described the ionic velocities were 

 very large, so that the density of electrification between the 



113 C 



plates was considerably reduced. The equation ==c?^, 



as shown above, was found to be true over a wide range of 

 X, c, and P. The mobility of the negative ion was shown 

 to be a function of the electric force only, and not to change 

 with the distance between the plates, which would be the 

 case if this source of error was great. 



4. The platinum strip freshly coated with salts gives off, 

 when heated for the first time, a considerable amount of 

 smoke (consisting probably of charged particles), which 

 make the results inconsistent. In this case the strip has to 

 be strongly heated at reduced pressure and in a strong 

 electric field before measurements are taken. 



Electric Wind in case of Ionization of both signs. 



It seemed to be of interest to study the pressure of the 

 electric wind in the case of ionization of both signs, when 

 the wind is produced by positive and negative ions moving 

 in opposite directions. For this purpose a small cell con- 

 taining 25 mgr. of radium bromide and provided with a thin 

 mica window was placed in the centre of the plate A in 

 place of the platinum strip, and the gas between the plates 

 strongly ionized by the a rays. The plate A could be 

 positively or negatively charged, the plate B being earthed, 

 and the wind-pressure measured in the usual way by the 

 gauge. In this case the pressure of the wind at any point 

 between the plates is a resultant of two opposite forces 

 produced by the motion of positive and negative ions. 



