[en] Within the A4 experiment the contributions of the strange quark to the electromagnetic form factors of the proton are measured. These see-quark effects in low energy observables are very important for the understanding of hadron structure, because they are a direct manifestation of QCD degrees of freedom in the non-perturbative regime. Linear combinations of the strangeness vector form factors of the proton (G_E^s and G_M^s) are accessible experimentally by measuring the parity violating asymmetry in the cross section of the elastic scattering of longitudinal polarised electrons off unpolarised nucleons. Two such measurements were published by the A4 collaboration before this work. Both of them were forward angle measurements at the Q² values of 0.23 and 0.10 (GeV/c)², respectively. A measurement at backward angle with a beam energy of 315 MeV was performed for separating G_E^s and G_M^s at the higher of these Q² values. In the A4 experiment a longitudinally polarised electron beam scatters on a liquid hydrogen target. Single scattered electrons are counted with a Cherenkov calorimeter. The separation of elastic from inelastic events is achieved by means of calorimetric energy measurement. For the backward angle measurement a plastic scintillator was installed as electron tagger for suppressing the γ background coming from the decay of π⁰ mesons. In order to make the data analysis possible the energy spectra needed to be studied thoroughly. This was done in this work using detailed simulations of both the scattering processes suffered by beam electrons and of the response of the detectors. A method for handling the remaining background due to γ conversion before the scintillator has been also developed. The simulation results agree with the measured spectra at the 5% level and the strategy for handling the background was shown to be feasible. The asymmetry value obtained by handling the background as proposed in this work was combined with the previous A4 forward angle measurement at the same Q² for separating G_E^s and G_M^s at Q²=0.22∝(GeV/c)². The results are G_M^s= -0.14 ± 0.11_exp ± 0.11_theo and G_E^s= 0.050 ± 0.038_exp ± 0.019_theo, where the systematic uncertainty due to the background correction is contained in the experimental error. Conclusions about the influence of strangeness on the static electromagnetic properties of the proton are drawn from these results and are presented at the end of the work. (orig.)