CONTEMPORARY ADVANCES IN PHYSICS 



127 



the particles being classifiable into groups each with its characteristic 

 speed. But if in such a curve there is a long sloping arc (as in Fig. 9), 

 it implies the analogue of a continuous spectrum, there being particles 

 of all ranges over a notable interval. 



The "stopping" or "absorbing" screens which are used in deter- 

 mining these curves are usually sheets of mica or of aluminium. 

 The curves are not however plotted against the actual thickness of 

 the interposed strata of mica or whatever else the substance may be, 

 but against the "air-equivalent" or thickness of the stratum of air of 

 standard density ^^ which is known by separate experiments to have 

 the same effect in slowing down and stopping charged particles, the 



-5 



JO 250 

 )(r 



:o 



^5 200 



Oct 150 



(Tq. 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 



AIR EQUIVALENT IN CENTIMETERS 



Fig. 9 — Integral distribution-in-range curve of the fragments resulting from bombard- 

 ment of lithium by deutons. (Oliphant Kinsey & Rutherford) 



same "stopping-power." It is the air-equivalent which is the quantity 

 X of the preceding paragraphs and the abscissa (often termed "ab- 

 sorption") of Fig. 8 and nearly all other such figures. The ratio 

 between the actual thickness of a layer of matter and the equivalent 

 thickness of air is roughly (but only roughly) the reciprocal of the 

 ratio of their densities. The sheets of metal or of mica used in the 

 experiments are therefore very thin (it has been possible to make 

 screens of mica so tenuous that their air-equivalent is only 0.15 mm.) 

 and the thinnest must be bolstered up by stiff metal grids, of which 

 the wires block a considerable fraction of the beam. It is also possible 



'^ There are unluckily two standards of density, one being that of air at 0° C. 

 and 760 mm. Hg, the other that of air at 15° C. and 760 mm. Hg; sec "Transmu- 

 tation," footnote on p. 643, B. S. T. J., Oct. 1931. The latter is used in this article. 



