  The hot gas in clusters of galaxies creates a distinctive spectral distortion
in the cosmic microwave background (CMB) via the Sunyaev-Zeldovich (SZ) effect.
To first order, the shape of the spectral distortion is fixed, but relativistic
corrections (rSZ) introduce a dependence on the gas temperature. In this paper,
we extract fluxes from a sample of 47 clusters in the Planck maps and make a
~5$\sigma$ detection of the rSZ effect by measuring the scaling relation
between the SZ amplitude (a proxy for cluster mass) and the cluster
temperature. Our measurement requires no prior knowledge of the clusters' gas
temperatures and hence is an example of how the rSZ can be used to probe
fundamental astrophysics. We find excellent agreement between our measurement
and temperatures obtained with X-ray measurements.
