442 Prof. F. Horton and Miss Doris Bailey on Measurement 



With this form of apparatus, a magnetic field parallel to the 

 axis of the tube was used to prevent the electron stream 

 from spreading laterally. All the metal (except the tungsten 

 filament) was platinum and had been boiled for several days 

 in strong nitric acid before being used. In every case the 

 complete apparatus was baked for many hours, and at the 

 same time kept evacuated to a low pressure by means of a 

 mercury-vapour pump; the filament was also heated to a 

 high temperature to rid it of occluded gases. During this 

 process mercury vapour from the pump was prevented from 

 passing over by an intervening U-tube containing carbon 

 cooled in solid carbon dioxide. In filling the experimental 

 apparatus with helium, the purified gas was allowed to pass 

 very slowly along a fine capillary tube and then through 

 the charcoal tube which was cooled in liquid air. 



The electric currents between the various electrodes were 

 measured by sensitive moving-coil galvanometers, while the 

 potential difference accelerating the primary electron stream 

 was gradually increased. It was found that the current due 

 to photoelectric action of the radiation on the electrodes was 

 too small in comparison with the original electron current to 

 be detected by the anode galvanometer, for when the apparatus 

 contained perfectly pure helium there was no sudden rise in 

 the current-E.M.F. curve until ionization of the gas occurred. 



On account of the velocity of emission of the electrons 

 from the filament and possibly of other causes, the measured 

 potential difference does not give the velocity of the electrons 

 passing through the gauze. In order to obtain the minimum 

 ionization velocity it is necessary to determine the correction 

 which must be added to the applied potential difference at 

 which ionization is first detected. For this purpose the type 

 of apparatus with two parallel grids has an advantage over 

 the type with one grid only; for with the former type it is 

 possible to obtain the correction to be added to the applied 

 potential difference so as to give the velocity of the electrons 

 which actually produce the effect measured by the anode 

 galvanometer, whereas with the single grid the correction 

 which is obtained gives the velocity of the swiftest electrons 

 present under a given applied accelerating potential difference, 

 irrespective of whether these swiftest electrons are sufficiently 

 numerous to produce a measurable amount of ionization when 

 their velocity reaches the critical value. 



A special investigation made with the apparatus repre- 

 sented in fig. 1 showed that the minimum number of electrons 

 which could be detected by the galvanometer could also 

 produce a detectable ionization current when their velocity 



