  LOFT (Large Observatory For x-ray Timing) is one of the four missions
selected in 2011 for assessment study for the ESA M3 mission in the Cosmic
Vision program, expected to be launched in 2024. The LOFT mission will carry
two instruments with their prime sensitivity in the 2-30 keV range: a 10 m^2
class large area detector (LAD) with a <1{\deg} collimated field of view and a
wide field monitor (WFM) instrument based on the coded mask principle,
providing coverage of more than 1/3 of the sky. The LAD will provide an
effective area ~20 times larger than any previous mission and will by timing
studies be able to address fundamental questions about strong gravity in the
vicinity of black holes and the equation of state of nuclear matter in neutron
stars. The prime goal of the WFM will be to detect transient sources to be
observed by the LAD. However, with its wide field of view and good energy
resolution of <300 eV, the WFM will be an excellent monitoring instrument to
study long term variability of many classes of X-ray sources. The sensitivity
of the WFM will be 2.1 mCrab in a one day observation, and 270 mCrab in 3s in
observations of in the crowded field of the Galactic Center. The high duty
cycle of the instrument will make it an ideal detector of fast transient
phenomena, like X-ray bursters, soft gamma repeaters, terrestrial gamma
flashes, and not least provide unique capabilities in the study of gamma ray
bursts. A dedicated burst alert system will enable the distribution to the
community of ~100 gamma ray burst positions per year with a ~1 arcmin location
accuracy within 30 s of the burst. This paper provides an overview of the
design, configuration, and capabilities of the LOFT WFM instrument.
