Rutherford's Experiment
I. January 1906
Rutherford announced the discovery of alpha particle scattering by air in January 1906. He took a wire coated radioactive material and passed the alpha rays through a narrow slit. This resulted in a narrow, rectangular beam instead of a narrow, circular pencil-shaped beam.
In a vacuum, the narrow slit showed perfectly sharp edges on a photographic plate the alpha rays hit. But, when the beam was passed through air, the edges of the slit became diffuse and widened out.
II. January to June 1906
Rutherford did not work in a vacuum, but instead discussed his results with his colleagues, including William H. Bragg. Bragg was not happy with Rutherford's conclusion and suggested an alternate explanation. Rutherford's response was to perform a more definitive experiment, publishing the results in June of 1906.
III. June 1906: Scattering by Mica
Rutherford used his alpha-emitting radioactive element covered with wire coating, but he modified the slit the alpha rays traveled through. Half the lit was left open and half was covered by a .003 cm) mica plate. The experiment was done in the vacuum with the mica taking the place of the air. The open side of the resulting photographic image was sharp and mica side was diffuse.
He also subjected the alpha beam to a magnetic field. This was done in response to Bragg's alternative explanation which involved electrons. Since electrons bend the opposite way from alpha particles, any electrons produced by the mica as the beam went through it would be swept away. The image that came out was the same as when there was no magnetic field. The vacuum side was sharp and the mica side was diffuse. Rutherford had once again proved that the alpha particles could be scattered.
IV. More results from the Mica Experiment
Careful measuring of the images allowed Rutherford to deduce that some alpha particles had been scattered by 2 degrees from a straight-line path. In light of future events, he makes the interesting statement that other particles may have been deflected through a considerably wider angle.
V. Manchester, 1908
While Rutherford was in Manchester he met a man named Hans Geiger. One of the central goals of Rutherford's work was to determine the nature of the alpha particle. Was it a singly-charged hydrogen molecule or a doubly-charged helium atom? The e/m ratio was consistent with either. Although in 1907, he was confident, he sought additional confirmation. To this end, while in Canada he had tried an experiment which needed to count the number of alpha particles. It failed. But, with Geiger now involved, a satisfactory counting device was designed and built.
However, the scattering of the alpha particle was wreaking havoc on their results. The counter worked great, but he said to a friend "the scattering is the devil."
As this work continued along, Geiger began to notice what he called a "notable" scattering. In; addition to this, he found that gold foil scattered through a greater angle than aluminum. He proposed to continue with as many metals as possible in hope of establishing some connection between the scattering and stopping powers of these materials.