How a cloud chamber works. The chamber holds air saturated with alcohol vapour above a very cold base, cooled by dry ice to about −78 °C. Just above the base the vapour is supersaturated: it is ready to condense but needs something to condense on. A charged particle passing through ionises air molecules along its path, and droplets of alcohol form on the ions, leaving a trail of mist.

Alpha trails. An alpha particle is strongly ionising, so its trail is thick and dense. It loses its energy quickly and stops after about 4 cm in air. Every alpha particle from americium-241 has the same energy, 5.5 MeV, so the trails are all the same length.

Beta trails. A beta particle is weakly ionising, so its trail is thin and faint, and collisions with air molecules knock it sideways. Beta particles are emitted with a range of energies. A beta particle can travel more than a metre in air, so most beta trails end when the particle leaves the thin layer of supersaturated vapour, not because it has stopped.

V-shaped trails. A thorium gas mantle gives off radon-220 gas. A radon-220 nucleus emits an alpha particle and becomes polonium-216, which has a half-life of 0.145 s and emits a second alpha particle. The two trails start from the same point, making a V.

Background radiation. Even with no source, trails appear: long, thin, straight trails from cosmic-ray muons, short wandering electron trails, and the occasional alpha trail from radon in the air.

The magnetic field. The force on a moving charge is at right angles to its velocity, so the particle moves along the arc of a circle. Positive and negative particles curve in opposite directions. Fleming’s left-hand rule gives the direction of the force; remember that the current is in the opposite direction to the motion of a negative particle.

The model. The ranges and momenta use real values for alpha and beta particles in air, and the beta momenta allow for relativity; p = BQr still holds. Real school chambers use permanent magnets of about 0.1 T or less; the slider goes higher so that the alpha trails can be seen to curve.