  In the standard model of solar flares, a large-scale reconnection current
sheet is postulated as the central engine for powering the flare energy release
and accelerating particles. However, where and how the energy release and
particle acceleration occur remain unclear due to the lack of measurements for
the magnetic properties of the current sheet. Here we report the measurement of
spatially-resolved magnetic field and flare-accelerated relativistic electrons
along a current-sheet feature in a solar flare. The measured magnetic field
profile shows a local maximum where the reconnecting field lines of opposite
polarities closely approach each other, known as the reconnection X point. The
measurements also reveal a local minimum near the bottom of the current sheet
above the flare loop-top, referred to as a "magnetic bottle". This spatial
structure agrees with theoretical predictions and numerical modeling results. A
strong reconnection electric field of ~4000 V/m is inferred near the X point.
This location, however, shows a local depletion of microwave-emitting
relativistic electrons. These electrons concentrate instead at or near the
magnetic bottle structure, where more than 99% of them reside at each instant.
Our observations suggest that the loop-top magnetic bottle is likely the
primary site for accelerating and/or confining the relativistic electrons.
