= Anderson positron track experiment
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The <Anderson positron track experiment> identified a light positively <charged particle>, now called the <Positron>, by combining magnetic bending, loss of <kinetic energy> in <lead>, and <particle range>. Each observation answered a different question: bending constrained <momentum>, the <lead> supplied a direction of travel, and <particle range> tested the proposed <mass>.
<Carl David Anderson>'s https://journals.aps.org/pr/pdf/10.1103/PhysRev.43.491[The Positive Electron] (1933), pp. 491–493, describes the 1932 discovery in a vertical <cloud chamber> with a $15{,}000\,\mathrm{gauss}$ <magnetic field>. A $6\,\mathrm{mm}$ <lead> plate separated two photographed track segments. The effective viewing depth was $1\,\mathrm{cm}$ along the <magnetic field>, with $14\,\mathrm{cm}$ transverse diameter: this restricted out-of-plane motion. The photographic readout supplied <curvature> on both sides. Comparisons with <Electron> tracks under the same conditions constrained <ionization>; specific <ionization> was not directly measured for these events. He tested accidental alignment of unrelated tracks and rejected a reversed <Electron> interpretation requiring an increase of <kinetic energy> in <lead>. Assuming unit <electric charge>, the <mass> bound was about twenty <Electron> masses, rather than an exact <mass> determination. The paper credits <Robert Andrews Millikan> with the design and <Seth Henry Neddermeyer> with assistance.<The Positive Electron by Carl David Anderson>[pp. 491–493, Fig. 1 and charge/mass discussion; p. 494, acknowledgment]{ref}
Preparation used rapid piston expansion and a <water>–<alcohol> mixture in the <cloud chamber>; cooling permitted droplets to form along <ionization> trails. A camera photographed these droplets. Initially, expansions occurred at random, so many photographs contained no tracks. Later counter triggering improved the useful yield.<Biographical Memoirs: Volume 73/Carl David Anderson by William Hayward Pickering>[Biographical Memoirs: Volume 73; William H. Pickering, Carl David Anderson, pp. 27–28: preparation, photographs and timing]{ref}
In https://www.nobelprize.org/uploads/2018/06/anderson-lecture.pdf[The Production and Properties of Positrons], his 1936 lecture, <Carl David Anderson> describes the chamber inside an <electromagnet> and explains why the <lead> plate was decisive: after losing <kinetic energy>, a particle bent more sharply. He describes a <proton> hypothesis predicting only $5\,\mathrm{mm}$ of <particle range>, against an observed path longer than $50\,\mathrm{mm}$. These are historical values from the lecture, not the stipulated example below. The point was the incompatibility of <particle range> with the <momentum> inferred from <curvature>, rather than positive <electric charge> alone.<Nobel Lectures, Physics 1922–1941/The Production and Properties of Positrons by Carl David Anderson>[Nobel Lectures, Physics 1922–1941; pp. 365–366, electromagnet, lead-plate direction test and proton range comparison]{ref}
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