  An instructional laboratory experiment to measure the muon mass is described.
Using coincidence-anticoincidence detection, the decay of a cosmic-ray muon
into an electron (or positron) is observed in a multiplate spark chamber, and
recorded with a triggered CCD detector. The energy of the charged decay-product
particle is then quantified by counting the number of spark gaps $n_S$ it
traverses before being stopped by the chamber's aluminum plates. By running
this apparatus under computer-control for several hours, the number of
product-particles $N(n_S)$ with various $n_s$-values is obtained. The muon mass
is obtained by a least-squares fit, which compares the experimentally observed
$N(n_S)$ with simulation values predicted for this distribution by the Fermi
description of muon decay via the weak interaction. We present the results for
the muon mass we have obtained and discuss the simulation we developed to
account for the observed skewing of $N(n_S)$ due to the various directions the
spark-producing product particles move as well as the escape of some of the
higher-energy particles from the chamber.
