The Board convened for its first meeting on 27 June 1958. Only four members were already engaged in space research: Rossi, Leo Goldberg, John Simpson, and James Van Allen. Rossi formed a subcommittee which included Thomas Gold, Philip Morrison and biologist Salvador Luria, who agreed that investigations of plasma in interplanetary space would be desirable. Consequently, Rossi decided to turn his group's efforts towards its study. With Herbert Bridge, Rossi designed and tested a plasma probe based on the classical Faraday cup. However, to enhance the instrument's response to positively charged protons and to suppress its response to photoelectrons produced by sunlight, four grids were deployed within the cup. A key innovation was a modulating voltage applied to one of the grids, which converted the signal into an alternating current, proportional to the proton flux and uncontaminated by any contribution of photoelectrons.