Editing Rutherford scattering experiments (section)

===Positive charge on nucleus: 1913===
In his 1911 paper ([[#1911 paper|see above]]), Rutherford assumed that the central charge of the atom was positive, but a negative charge would have fitted his scattering model just as well.<ref name=AIP3>{{harvnb|AIP}}</ref>  In a 1913 paper, Rutherford declared that the "nucleus" (as he now called it) was indeed positively charged, based on the result of experiments exploring the scattering of alpha particles in various gases.<ref name=RutherfordNuttal1913/>

In 1917, Rutherford and his assistant William Kay began exploring the passage of alpha particles through gases such as hydrogen and nitrogen. In this experiment, they shot a beam of alpha particles through hydrogen, and they carefully placed their detector—a zinc sulfide screen—just beyond the range of the alpha particles, which were absorbed by the gas. They nonetheless picked up charged particles of some sort causing scintillations on the screen. Rutherford interpreted this as alpha particles knocking the hydrogen nuclei forwards in the direction of the beam, not backwards.<ref name=AIP3>{{harvnb|AIP}}</ref>