  At the electron accelerator ELSA a linearly polarised tagged photon beam is
produced by coherent bremsstrahlung off a diamond crystal. Orientation and
energy range of the linear polarisation can be deliberately chosen by accurate
positioning of the crystal with a goniometer. The degree of polarisation is
determined by the form of the scattered electron spectrum. Good agreement
between experiment and expectations on basis of the experimental conditions is
obtained. Polarisation degrees of P = 40% are typically achieved at half of the
primary electron energy. The determination of P is confirmed by measuring the
beam asymmetry, \Sigma, in pi^0 photoproduction and a comparison of the results
to independent measurements using laser backscattering.
