CONTEMPORARY ADVANCES IN PHYSICS 643 



at 0° C. and atmospheric pressure.^ The foil of aluminium destined 

 to be transmuted stands either at L or at F. If at L, it receives the 

 full impact of the unretarded alpha-particles. If it is placed at F, one 

 or more foils of gold are located at L (one at least must be set there, so 

 as to prevent "contamination" of the chamber by atoms of radioactive 

 substances escaping from the layer of polonium and wandering around) 

 and these reduce the energy of the alpha-particles before they strike 

 the leaf of aluminium. At G are placed the tenuous sheets of mica 

 which retard or stop the protons, enabling the observer to plot the 

 aforesaid curve (and incidentally protecting the vacuum within the 

 upper chamber against the gas without). Below is the device for 

 counting the protons which have succeeded in passing the mica sheets. 

 This device for detecting protons is not a fluorescent screen, but a 

 conical chamber filled with carbon dioxide, in which the corpuscles 

 engender thousands of ions as they cross. Between the walls of the 

 cone and the wire which runs part way along its axis, there is a voltage 

 sufficient to draw ninety per cent of the negative ions to the one, of the 

 positive ions to the other. An electrometer connected to the wire gives 

 a kick whenever a corpuscle passes through ; the deflection is a measure 

 of the total charge borne to the wire by ions of one sign, therefore of the 

 total number of these. The kicks are not overly frequent; in cases 

 mentioned by Pose they amounted to thirty or thereabouts per hour. 

 They are not all equal; on the contrary, they range from almost im- 

 perceptible deflections (corresponding to 5000 ions or less) to a maxi- 

 mum which indicates seventy thousand. They are not all due to pro- 

 tons, for some are observed when the conical chamber is closed on all 

 sides; these are ascribed to alpha-rays emanating from radioactive 

 atoms which happen to be in the gas of the chamber or in the walls 

 thereof; they are counted in "blank" experiments, and a number equal 

 to theirs is deducted from the total number observed when the protons 

 from the aluminium are coming in. It appears from the data that all 

 of the corpuscles which produce more than 25000 ions apiece are of 

 this undesired type, while most of those which cause the smaller kicks 

 of the electrometer do actually come from the metal foil which is 

 suffering transmutation. 



Every detail of the set of curves next following (Fig. 6) is worth 

 examining. They are curves of the sort which I defined above, except 

 that the ordinate is not the actual number of protons observed, but the 

 quotient of this number by that of the alpha-particles expressed in 

 2 This is Pose's convention, to which I conform in the following pages; the range 

 of alpha-rays from polonium is 3.92 cm. in air at 15° C. and atmospheric pressure; the 

 other ranges mentioned in what follows should be increased m the same proportion if 

 the reader wishes to hold to Rutherford's convention. 



