Measurements of the electromagnetic form factor of the Proton at JLab

The ratio of the proton's elastic electromagnetic form factors $G_{Ep}/G_{Mp}$ was obtained by measuring $P_{t}$ and $P_{\ell}$, the transverse and longitudinal recoil proton polarization, respectively. For the elastic reaction $\vec e p \to e\vec p$, $G_{Ep}/G_{Mp}$ is proportional to $P_t/P_{\ell}$. The simultaneous measurement of $P_{t}$ and $P_{\ell}$ in a polarimeter reduces systematic uncertainties. The results for the ratio $G_{Ep}/G_{Mp}$ measured in Hall A so far show a systematic decrease with increasing $Q^2$, indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. Together these experiments cover the Q$^{2}$- range of 0.5 to 5.6 GeV$^{2}$. A new experiment is currently being prepared, to extend the Q$^2$-range to 9 GeV$^2$ in Hall C.