We have developed a highly sensitive 3 He magnetometer for the accurate measurement of the magnetic field in an experiment searching for an electric dipole moment of the neutron. By measuring the Larmor frequency of nuclear spin polarized 3 He atoms a sensitivity on the femto-Tesla scale can be achieved. A 3 He/Cs-test facility was established at the Institute of Physics of the Johannes Gutenberg University in Mainz to investigate the readout of 3 He free induction decay with a lamp-pumped Cs magnetometer. For this we designed and built an ultra-compact and transportable polarizer unit which polarizes 3 He gas up to 55% by metastability exchange optical pumping. The polarized 3 He was successfully transfered from the polarizer into a glass cell mounted in a magnetic shield and the 3 He free induction decay was detected by a lamp-pumped Cs magnetometer. PACS numbers 07.55.Ge Magnetometers for magnetic field measurements; 13.40 Electric and magnetic moments; 14.20 Protons and neutrons