The Discovery of the Beta Particle

Natural radioactivity was discovered by Henri Becquerel early in 1896. Only after three and a half years had passed was it determined that (at least) part of this radiation consisted of material particles. Towards the end of 1899 three different sets of investigators in as many countries found independently that radium rays were bent when they traversed a magnetic field. This paper delineates both the reasons for this delay and the events which converged in 1899 to make likely the simultaneous discovery of the deflection of the beta rays in a magnetic field. It also indicates the significance this discovery had for the future development of radioactivity.