[en] A new high resolution iron-core electron spectrometer (BILL) has been constructed at the High Flux Reactor in Grenoble. It is currently being used for measurements of conversion electrons after thermal neutron capture. An internal target (maximal size 40 cm²) is irradiated in a thermal flux of 3x10¹⁴n cm^-2sec^-1. The electrons pass through a vertical beam tube, 14 m long and 10 cm in diameter, defining the solid angle of 3.4x10^-6/4π. The spectrometer itself consists of two independent flat electromagnets. Both magnets act as double focussing spectrometers, where the double focussing is achieved by the combination of homogeneous and 1/r fields. The first, a 58⁰ deflecting magnet, forms a 10:1 scaled down intermediate image of the large, 14 m distant target in the reactor hall. The dispersion of this magnet is 94 cm. The second magnet views this reduced image with a very high dispersion of 540cm. The radius of the optic circle of both magnets is 50cm. The energy range of the spectrometer is 15 keV< E<10 000 keV. Multi-wire proportional counters are used as detectors. The best measured momentum resolution has been 8x10^-5 at 300 keV with a target of 2x100mm². Routine measurements have been carried out with targets of 30x100 mm² achieving a resolution of Δp/p=4x10^-4. The momentum precision of the instrument is 1 part in 10⁵ and the sensitivity for neutron capture conversion electrons 300μb at 400 keV. (Auth.)