 Rotating-wheel molecular velocity selector

Otto Stern's lecture describes two co-rotating toothed wheels: transit between them must match an advance between openings. He explicitly distinguishes this method from the first historical speed measurement.^[ref]

For an idealized example, let aligned wheels have $N$ equally spaced openings, separation $L$, and rotation rate $f$ in revolutions per second. A particle with constant longitudinal speed $v$ takes $L/v$ between them. The second wheel advances through $fL/v$ revolutions; transmission peaks when that advance is an integer number $k$ of opening pitches:With chosen $N=60$, $f=50\mathrm{Hz}$ and $L=0.080\mathrm{m}$, the first two orders select $240$ and $120\mathrm{m}{\mathrm{s}}^{-1}$. A second transmission maximum therefore need not represent a second source temperature. Finite opening widths admit speed intervals around each order, and an additional measurement is needed to distinguish orders. The ideal expression assumes negligible acceleration and constant rotation rate during a flight, with a fixed relative phase between the wheels.