of the Ionization Velocity for Electrons in Helium. 443 



was above the critical value, so that, using a galvanometer as 

 the current-measuring instrument, the two "corrections" re- 

 ferred to above are identical. The experiments were made 

 ;is follows: — A potential difference Vi was applied between 

 the filament and the first grid in such a direction as to oppose 

 the emission of electrons from the filament. By the variation 

 of this field the number of electrons passing through the 

 gauze could be controlled. In order to determine for what 

 value of the potential difference Vj the minimum number of 

 electrons capable of giving a current measurable with ihe 

 anode galvanometer was obtained, the electrons after passing 

 through the first gauze were accelerated by the fields V 2 and 

 V 3 , towards tlie collecting electrode, the sum of the potential 

 differences V._> and V 3 being less than the ionization potential 

 difference. While V 2 and V 3 remained constant. Y 1 was 

 gradually reduced from a value too great to allow any 

 electrons to pass through the first gauze, until the anode 

 galvanometer gave the first indication of a current passing 

 through it. The value of \\ at which this first indication was 

 obtained is the correction to be applied to give ihe velocity of 

 the swiftest electrons present. 



In the second part of the experiment, the field Y 1 con- 

 trolled the number of electrons employed, but the potential 

 difference Y 2 was made a little above the ionization value. 

 so that the electrons which passed through the first gauze 

 attained sufficient velocity between the first and second 

 gauzes to be able to ionize helium atoms on collision with 

 them in the space above the second gauze. The potential 

 difference V 3 in this space opposed the electrons from the 

 filament and was maintained sufficiently great to prevent 

 any of them from reaching the collecting eleefrode. As the 

 photoelectric effect of the radiation on the electrodes does 

 not produce a measurable current, the only current measured 

 by the anode galvanometer is that due to the positive ions 

 produced in the space between the second gauze and the 

 collecting electrode, by the impact of the electrons with 

 Indium atoms. Keeping the potential differences V 2 and V 3 

 constant, Vj was adjusted to the limiting value at which a 

 detectable ionization current was measured by the galvano- 

 meter. It was found that this limiting value of V] was the 

 same as the value of V t obtained in the first part of the 

 experiment. Thus it is safe to assume that when a single- 

 grid valve was used, the ionization velocity deduced from the 

 swiftest electrons detectable, was actually the velocity of those 

 which first produce a measurable ionization. 



In investigating the ionization velocitv for electrons in 



2 G 2 



