  In this paper we report 23 magnetic field measurements of the B3IV star HD
23478: 12 obtained from high resolution Stokes $V$ spectra using the ESPaDOnS
(CFHT) and Narval (TBL) spectropolarimeters, and 11 from medium resolution
Stokes $V$ spectra obtained with the DimaPol spectropolarimeter (DAO). HD 23478
was one of two rapidly rotating stars identified as potential "centrifugal
magnetosphere" hosts based on IR observations from the Apache Point Observatory
Galactic Evolution Experiment survey. We derive basic physical properties of
this star including its mass ($M=6.1^{+0.8}_{-0.7}\,M_\odot$), effective
temperature ($T_{\rm eff}=20\pm2\,$kK), radius
($R=2.7^{+1.6}_{-0.9}\,R_\odot$), and age ($\tau_{\rm age}=3^{+37}_{-1}\,$Myr).
We repeatedly detect weakly-variable Zeeman signatures in metal, He and H lines
in all our observations corresponding to a longitudinal magnetic field of
$\langle B_z\rangle\approx-2.0\,$kG. The rotational period is inferred from
Hipparcos photometry ($P_{\rm rot}=1.0498(4)\,$d). Under the assumption of the
Oblique Rotator Model, our obsevations yield a surface dipole magnetic field of
strength $B_d\geq9.5\,$kG that is approximately aligned with the stellar
rotation axis. We confirm the presence of strong and broad H$\alpha$ emission
and gauge the volume of this star's centrifugal magnetosphere to be consistent
with those of other H$\alpha$ emitting centrifugal magnetosphere stars based on
the large inferred Alfv\'en to Kepler radius ratio.
