| In 1912, C.T.R, Wilson of Cambridge University succeeded
in photographing the tracks made by alpha particles in supersaturated water
vapor. He used a technique that he developed fifteen years earlier in which
the vapor is suddenly expanded, leading to cooling and supersaturation.
The cloud chamber, listed at $45 in the 1922 catalogue of the Central Scientific Company, is shown at the right. The catalogue copy reads: "The expansion chamber consists of a glass ring, above which is clamped a circular disk of plate glass. The lower end of the ring fits into a metal casting, shaped like an inverted cone, to the lower end of which a rubber bulb is attached The bulb and inverted cone contain water up to a point about 1 cm below the plate glass surface. In the air space of the expansion chamber is mounted a glass tip at the end of which, in a protected cavity, there is a small amount of radium salt. The air space is illuminated by an enclosed electric light, the housing of which has a shielded slit opposite the air space. With an electrostatic field of 100 to 200 volts across the air space the bulb is compressed and suddenly released. The expansion of the air space incident to releasing the bulb causes the tracks of the alpha particles to become visible as a result of the condensation of moisture upon the ions produced by the alpha particles."
This "Knipps Apparatus" is in the Greenslade Collection.
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