Other particles were discovered around this time called alpha rays. They were positively charged and it was thought that they were the positive parts of the Thompson atom (it's now known to be the nucleus of atoms).
In 1911 Ernest Rutherford thought it would be interesting to bombard atoms with these alpha rays, figuring that it could be used to investigate the inside of the atom (sort of like a probe). He used Radium as the source of the alpha particles and shinned them onto the atoms in gold foil. Behind the foil was a fluorescent screen for which he could observe the alpha particles impact.