In this paper

we describe the principles and structure of the beam position monitor(BPM)

which is composed with two thin tungsten wires perpendicular to each other.A linear motion forming 45 ° with respect to horizontal scans simultaneously two wires across the beam spot to measure both photoemission currents as function of the position.The BPM has been mounted on the XAFS beamline at NSRL to monitor the synchrotron radiation stability from the Wiggler

including the photocurrents distribution in vertical and horizontal directions and drifts of the beam spot as attenuation of the electronic beam in different injections.It is useful in improving operation of the storage ring and collimating optical elements of the beamline.