Demonstration Models of Physical Processes

Abstract

Two interesting models, originally designed for exhibition in the Science Museum, London, have been described by Dr. F. A. Ward (Proc. Phy&. Soc.9 Jan. 1947). The first illustrates, by an optical analogy, the principle of the Bragg X-ray spectrometer, namely, that a monochromatic beam of X-rays will be reflected by a single crystal only when the crystal is set at a particular angle to the incidenrt beam such that reflexions from successive sheet rich in atoms are in phase and so give an interference maximum. The incident beam is represented by a sheet of corrugated iron, and the crystal to be analysed by four glass plates mounted on a wooden rack. The observer looks into the glass plates, and rotates them until the virtual images of the crests of the corrugations appear to coalesce. The second model is a mechanical one illustrating the uranium chain reaction which has been utilized for the release of atomic energy. It consists of thirty units, each representing the nucleus of an atom of uranium 235 or plutonium. A unit consists of two portions held together by a catch which, when released, allows the two portions to fly apart, while at the same time projecting upwards three table-tennis balls representing neutrons. A single ‘neutron’ dropped on the catch of one of the units disintegrates this unit, and the three ejected ‘neutrons’ impinging on neighbouring units cause them to disintegrate, thus initiating the chain reaction. Both these models have been demonstrated at the Physical Society's science meetings, at the Society's Exhibition of Scientific Instruments and Apparatus, and also at the recent Royal Society conversazione, where they have aroused considerable interest.