The operation characteristics of a wire-plane corona streamer detector (WPCSD) in open air were investigated for broad side irradiation. Investigations were made on the counting characteristics in relation to their dependence on various anode diameters and source-anode spacing. The onset voltages for the corona mode Vo as well as for the spark mode Vp were determined. For each wire diameter a streamer mode (Vp - Vo) was found. The sensitivity of the WPCSD was studied when a collimated beam of a-particles was moved parallel to the plane of the cathode from the anode-wire toward the cathode. In addition, the pulse height spectra of alpha-particles incident on the detector for different values of the anode voltage, Va, were determined and studied. A qualitative interpretation of the experimental data was given.