458 Mr. S. Ratner on the 



P X 



Equation (1) may be written in the form — =d— , which 



shows that the wind-effect produced by the ions is inversely 

 proportional to their mobility. According to Wellisch's 

 theory the wind-pressure measured by the gauge in these 

 experiments is mostly due to the slow normal ions, the effect 

 produced by the free electrons being very small. The 



P 



ratio — , as shown above, diminishes with increasing electric 



c 



forces, and reaches in some experiments 5 per cent, of its 

 initial value. It is necessary therefore to suppose that 

 the proportion of free electrons increases with the electric 

 force, so that, finally, the amount of normal ions becomes 

 vanishingly small. Wellisch, however, states that the pro- 

 portion of free electrons is independent of the strength of the 

 field, and is always less than 50 per cent, of the total number 

 of the negative carriers. 



In order to verify the results of W ellisch, experiments were 

 carried out which were based on the principle of separating, 

 bj r means of an alternating electric field, the normal ions 

 from the free electrons, and measuring the wind-effect due 

 to one of these two kinds of ions separately. It appeared 

 necessary for this purpose to study at first the wind-effect 

 produced by the negative ions in an alternating field under 

 conditions in which the mobility is normal, i. e. when, 

 according to Wellisch, all the ions are normal ions. The 

 alternating field between the plates A and B was established 

 by means of a high-speed commutator which could pro- 

 duce up to 2000 alternations per second *, the potential 

 difference between the plates being kept constant. The 

 experiments show that when the number of alternations is 

 gradually increased, the wind-pressure at first slowly dimin- 

 ishes and then rapidly falls down to a very small value. If 

 the wind-pressure is plotted against the number of alterna- 

 tions per unit time, the curves obtained are all of the type 

 of curve ( + ) (fig. 8), and simple calculations show that the 

 sudden drop in the wind-pressure corresponds to the number 

 of alternations at which the ions emitted by the heated strip 

 begin not to reach the grating. 



Now, if we repeat the same experiments under the con- 

 ditions, in which the mobility of the negative ion is abnormal, 

 i. e. when, according to Wellisch, the wind-pressure is mostly 

 produced by the normal ions and only to a small extent by 

 the free electrons, the following observations are to be 



* The commutator was that used by G. Todd, Phil. Mag. xxii. p. 791 

 (1911). 



