Much of the experimental data underpinning the Standard Model of particle physics has been collected at particle accelerators. The exponential increase in the energy of these machines has recently begun to slow, as the cost and scale of the projects reach historic levels. New accelerating technologies may offer a way to reverse this trend and provide smaller scale or higher energy accelerators. One such novel acceleration method is proton-driven plasma wakefield acceleration and the AWAKE experiment at CERN has been conceived to provide proof of this new acceleration principle. A 400 GeV proton bunch is used to drive a wakefield in a 10 m column of plasma. Low energy electrons are injected into this wake and accelerated to high energies. This accelerated electron bunch is diagnosed with a magnetic spectrometer. This spectrometer has been designed, installed and calibrated in order to provide measurements of the energy and charge of the accelerated electron bunches at AWAKE.