694 BELL SYSTEM TECHNICAL JOURNAL 



Dempster experimented with a scheme which may be likened to 

 Ramsauer's with the final slit halfway around the circle from the initial 

 slit, and all the slits and walls between suppressed. ^^ Later it was 

 adopted by some of his pupils, and by Kallmann and Rosen. Various 

 mixtures were used by Dempster and his pupils to produce the ions; 

 some were heated, some bombarded by electrons; one of them, when 

 thus bombarded, emitted protons and Ho"^ ions, a very useful 

 property. When one varies the magnetic field and plots against it the 

 current which passes through the final slit and is captured by an elec- 

 trometer posted behind, one gets a curve with a series of peaks, one for 

 each kind of ion present. 



When the deflection-chamber is well evacuated, the peaks are sharp 

 and narrow; when gas is introduced, they become lower and broader, 

 and sometimes they are visibly displaced. In certain cases (K+ ions in 

 helium, for instance) the height of the peak falls off exponentially with 

 increase of pressure, and one deduces the value of cr by equation (9) ; 

 it invariably turns out to be smaller than the value one gets by sup- 

 posing both the ions and the atoms to have their gas-kinetic cross- 

 sections, and decreases as the speed of the ions is increased. The 

 data are thus in qualitive agreement with those of Ramsauer and Beeck, 

 but a thoroughgoing comparison is yet to be made. The broadening 

 of the peaks is a sign that the ions in their encounters with the particles 

 of the gas are suffering small deflections with scarcely any loss ol 

 energy; the displacement of the peaks (when it occurs) a sign that they 

 are losing small amounts of energy. A simple reduction in the height 

 of a peak, unattended by broadening or displacement, might well 

 indicate that the electron-transfers far outweigh the other modes of 

 interception, and that the value of a obtained is the a for electron- 

 transfer. 



The value of a for protons, Dempster found, is remarkably small. 

 One may visualize these hydrogen nuclei as mere points, as one does an 

 electron, and consider a as the "cross-section for interception of 

 protons" of molecules of the gas. Sending 900-volt protons through 

 helium, in apparatus of the type above, Dempster observed the 

 peak surviving even when the density of the gas was so great, that 

 if o" had been equal to the gas-kinetic cross-section ao, more than half 

 of the ions would have been intercepted in the first one-hundredth of 

 their semi-circular path. It was considerably broadened, as though 

 most of the protons had suffered small deflections; but at a density as 

 much as one-seventh as great, there was hardly any broadening even ; 



2' I must not give the impression that Dempster's scheme was a copy of Ramsauer's 

 or Smyth's; it antedated both, having been used for other purposes, e.g. the detection 

 of isotopes. 



