  Using data from the WiFeS Atlas of Galactic Globular cluster Spectra we study
the behaviour of the calcium triplet (CaT), a popular metallicity indicator in
extragalactic stellar population studies. A major caveat of these studies is
that the potential sensitivity to other stellar population parameters such as
age, calcium abundance and the initial mass function has not yet been
empirically evaluated. Here we present measurements of the strength of the CaT
feature for 113 globular clusters in the Milky Way and its satellite galaxies.
We derive empirical calibrations between the CaT index and both the iron
abundance ([Fe/H]) and calcium abundance ([Ca/H]), finding a tighter
relationship for [Ca/H] than for [Fe/H]. For stellar populations 3 Gyr and
older the CaT can be used to reliably measure [Ca/H] at the 0.1 dex level but
becomes less reliable for ages of $\sim 2$ Gyr and younger. We find that the
CaT is relatively insensitive to the horizontal branch morphology. The stellar
mass function however affects the CaT strengths significantly only at low
metallicities. Using our newly derived empirical calibration, we convert our
measured CaT indices into [Ca/H] values for the globular clusters in our
sample.
