J.J. Thomson's positive ray apparatus, replica of Cavendish Lab apparatus


J.J. Thomson's positive ray apparatus, replica of Cavendish Lab apparatus. Object ID EM.N-08014; Overall length 62 cm x width 35 x height 44 cm.

Object is a replica of the original which resides at Cavendish Laboratory, Cambridge University, England. Wood base supports, at one end, a horizontal glass discharge tube attached to a chamber fitted between the poles of an electromagnet. The chamber contains a pair of plates for deflection of the beam of "positive rays" (ions) passing through from the discharge tube. The rays fall on a phosphorescent screen at the other end of the apparatus, where a photographic plate can also be positioned.

Basic Principle

Thomson's positive ray apparatus were analogous to his cathode-ray tubes (see objects EM.N-08013A and EM.N-08013B); in that they measured the ratio of charge to mass of "positive rays" instead of electrons (cathode rays).

Ions, created by an electric discharge in the glass bulb, are accelerated towards its mouth. This beam of "positive rays" is then deflected vertically by a magnetic field and horizontally by an electric field, forming a parabolic trace on a phosphorescent screen—all atoms or molecules of the same charge-to-mass ratio falling along one parabola. This object is a replica of the fourth such apparatus Thomson constructed. With it, the traces were recorded photographically for the first time. For a concise review of J.J. Thomson, the Cavendish Laboratory, and Thomson's cathode ray tube and positive ray apparatus, see J .J. Thomson - the Centenary of His Discovery of the Electron and his invention of Mass Spectrometry, Rapid Communications in Mass Spectrometry, Vol.11, 2-16 (1997).

Date Made: ca. 1960 (original 1907-1912)

Manufacturer: University of Cambridge. Department of Physics. Cavendish Laboratory

Location: Currently not on view

Place Made: United Kingdom: England, CambridgeAssociated Place: United Kingdom: England, Cambridge

General Subject Association: Science & Scientific InstrumentsModern Physics


See more items in: Medicine and Science: Modern Physics, Science & Mathematics


Exhibition Location:

Credit Line: Cavendish Laboratory

Data Source: National Museum of American History

Id Number: EM.N-08014Accession Number: 224580

Object Name: Positive Ray Apparatus (replica)positive ray apparatus (replica)positive ray apparatus (replica)Electron Tube, Positive Ray Apparatus

Physical Description: wood; glass; metal (overall material)Measurements: overall: 44 cm x 35 cm x 62 cm; 17 5/16 in x 13 25/32 in x 24 13/32 in

Guid: http://n2t.net/ark:/65665/ng49ca746a8-8a41-704b-e053-15f76fa0b4fa

Record Id: nmah_1004733

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