the authors report on a series of experiments with a superconducting 500 mhz single cell cavity of 'spherical' shape fabricated from niobium sheet material. design, fabrication and different room temperature surface treatments of cavity including ion bombardment cleaning are discussed. an experimental test set-up was used, which includes diagnostic systems. the following general characteristics of the cavity were observed' at 4.2k an accelerating field of 3 mv/m can be achieved safely with low losses (q=2*10/sup 9/: p/sub diss/=8.4 w/m). the residual rf losses are concentrated in the high electric field region of the cavity surface and cannot be attributed to normal conducting metallic impurities, but show dielectric loss properties. resonant electron loading was never observed. at field levels exceeding 3 mv/m nonresonant electron currents start to load the cavity q and give rise to the excitation of higher order cavity modes. the electron sources are pointlike and located in the high electric field region of the cavity. the field emission nature and the location of the electron sources are determined by temperature mapping and x-ray diagnostic measurements and analyzed by computer simulation of electron trajectories. in the region between 4 and 5 mv/m the cavity field is limited by thermal instabilities originating from pointlike loss regions predominantly found at the bottom part of the cavity. some of these defects were observed to be created during high field operations of the cavity in several instances