. Il giornale tecnico Bell System . ntando tutti gli ioni a butquelle di un intervallo di velocità molto stretto dal raggiungimento della camera di deflessione. Il dispositivo Bainbridge per quest'ultimo scopo è mostrato in Fig. 3; tra le piastre del filtro di velocità un campo elettrico trasversale è sovrapposto al campo magnetico che è ad angolo retto rispetto al piano della carta, e nessuna particella carica passa fino al lit a meno che la sua velocità sia molto quasi uguale al rapporto delle intensità di campo. Se un fascio di ioni tutti di massa M identica e carica e e velocità v 310 BELL SYSTEM TECHNICAL JOURNAL dovevano entrare nella camera
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. The Bell System technical journal . nting all ions butthose of a very narrow velocity-range from reaching the deflection-chamber. Bainbridges device for this latter purpose is shown inFig. 3; between the plates of the velocity-filter a transverse electricfield is superposed on the magnetic field which is at right angles tothe plane of the paper, and no charged particle gets through to theslit unless its speed is very nearly equal to the ratio of the field-strengths. If a beam of ions all of identical mass M and charge e and speed v 310 BELL SYSTEM TECHNICAL JOURNAL were to enter the chamber through the sHt they all would follow thesame semicircle and assemble on the very same spot on the plate, the distance of which spot from the slit would tell the observer theirmass. But when a beam of ions of a single element is projectedthrough the slit, it is not usually a single spot which appears uponthe plate. All students of physics have seen reproductions of suchplates, chiefly from Astons magnificently ample store. I reproduce. Fig. 3—Scheme of Bainbridges apparatus for accurate measurement of the masses of isotopes. here two from Bainbridges, Fig. 4 for zinc and Fig. 5 for germanium.These are mass-spectra every spot or line of which is the evidenceof a separate isotope of the element in question. Germanium andzinc are neither the least nor the most profuse in isotopes among theelements; there are still a few (fluorine and sodium, for instance) forwhich only one has been discovered, and at the other extreme there istin with no fewer than eleven. It is, of course, the charge-to-mass ratio of the ion rather than itsmass which is deduced from the position of the spot and the strengthsof the accelerating and deflecting fields. (There is no need of givingthe formula here, as it is to be found in every textbook and is readily CONTEMPORARY ADVANCES IN PHYSICS 311 derived.) The charge is usually + e (singly-ionized atom), sometimes+ 2e (doubly-ionized atom), rarely + 3e or gre
