  The local magnetic field in a Penning-Malmberg trap is found by measuring the
temperatures that result when electron plasmas are illuminated by microwaves
pulses. Multiple heating resonances are observed as the pulse frequencies are
swept. The many resonances are due to electron bounce and plasma rotation
sidebands. The heating peak corresponding to the cyclotron frequency resonance
is identified to determine the magnetic field. A new method for quickly
preparing low density electron plasmas for destructive temperature measurements
enables a rapid and automated scan of microwave frequencies. This technique can
determine the magnetic field to high precision, obtaining an absolute accuracy
better than $1\,\mathrm{ppm}$, and a relative precision of $26\,\mathrm{ppb}$.
One important application is in situ magnetometry for antihydrogen-based tests
of charge-parity-time symmetry and of the weak equivalence principle
