For investigation of a multipoint explosive emission cathode operation in an external magnetic field an experimental electron source with a grounded cathode was developed. It allows to carry out measurements of currents through the cathode points and probe measurements of the cathode plasma parameters. The following main results were obtained. An increase of the magnetic field leads to an increase of the intensity of the cathode plasma generation and hence to a decrease of the pulse duration. The maximum pulse duration is achieved in the absence of a magnetic field. The pulse duration depends also on the magnitude of the resistance in the circuit of the cathode mask Rm. Optimum pulse duration is achieved for R_m > 15 Ω. Increasing R_m up to several kΩ leads to an additional increase of the pulse duration by 15%. At the front of the pulse only around 30% of the cathode fibers operate for any magnitude of an accelerating electrical field. The dynamics of the process of the fiber ignitions is determined by the magnitude of the electric field in the accelerating gap. At a maximum (achievable at the facility used in this experiments) electric field strength of 17-18 kV/cm practically all fibers start to operate within 3 us after beginning of the high voltage pulse. The magnetic field does not influence the fiber ignition behavior. The difference between the currents through the cathode fibers is 15-20%. In the absence of a magnetic field the inhomogeneity of the Plasma density does not exceed 5%. This testifies good homogeneity of the cathode plasma density, in despite of some inhomogeneity of the currents through the fibers. When a magnetic field is applied, the nonuniformity of the Langmuir probe currents increases up to 15%. Therefore, the magnetic field influences the distribution of the cathode plasma density. Accelerating voltage practically does not influence the homogeneity of the cathode plasma density.