= Bubble-chamber track readout
<Stereoscopy> enables three-dimensional <track reconstruction>.<How It Works: Bubble Chamber (CERN teaching archive)>[How it works, views]{ref} Calibrated <camera> positions and optical geometry are needed before a photographed arc becomes a physical <radius of curvature>. <Neutral particles> do not directly leave the same continuous <ionization> trail as <charged particles>; an interaction or decay can instead reveal them through charged secondary tracks. A <photon>'s inferred path between interactions is not a photographed luminous line.
In a uniform <magnetic field>, with negligible <stopping power> and <elastic scattering> over the fitted segment, the transverse projection is a <circle>. The <Lorentz force> turns transverse <momentum> at angular rate $v_\perp/r$, giving
$$
\frac{p_\perp v_\perp}{r}=|q|v_\perp B,\qquad
p_\perp=|q|Br.
$$
Here $r$ is measured in the plane perpendicular to the <magnetic field>. The relation remains valid with relativistic <physical momentum>; it does not require $p=mv$. Thus curvature determines <magnetic rigidity>, not <mass> or <kinetic energy> alone. For chosen $|q|=e$, $B=1.5\,\mathrm T$ and $r=0.40\,\mathrm m$, $p_\perp\simeq0.180\,\mathrm{GeV}/c$. This is an invented example, not a fit to a historical photograph. Reconstructing a three-dimensional <helix> also determines the component parallel to the <magnetic field>; the transverse <radius of curvature> alone does not. Determining the sign of <electric charge> additionally requires the field direction and the direction of travel, as in the <charge-sign ambiguity of a curved track>.
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