University of Cambridge. Department of Physics. Cavendish Laboratory Search this
Physical Description:
glass; metal; wire (overall material)
Measurements:
overall: 30.5 cm x 8.5 cm; 12 in x 3 11/32 in
Object Name:
Electron Tube, Cathode Ray
electron tube, cathode ray (replica)
Associated Place:
United Kingdom: England, Cambridge
Date made:
ca. 1960
Description:
J.J. Thomson's cathode ray tube #2, replica of Cavendish Lab apparatus. Object ID EM.N-08013-A; overall; length 30.5 cm, glass ball diam. 8.5 cm.
This object consists of a glass cylindrical body with a larger diameter glass ball at one end. Four shorter glass tubes with internal wires extend perpendicularly from the cylinderical body (see 1st object on left in accompanying image).
Basic principle
Rays from cathode in cylinder pass through 2 metal slots and between 2 metal plates onto glass ball. Metal plates can have electric field applied across them. External magnetic field can be applied to balance electric deflection. Used to determine the ratio of charge to mass (q/m) of the electron. For theory Ref: J.J Thomson, Phil. Mag., 44 (1897) 293. For a concise review of J.J. Thomson, the Cavendish Laboratory, and Thompson'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).
History
In the second half of the nineteenth century the phenomena accompanying the discharge of electricity through highly rarified gases received increasing attention from physicists. The development of theory and experimental technique raised expectations of insight from this quarter into the structure of the atom and the connection between matter and electricity. (See object ID EM.N-08253, Geissler tube demonstration apparatus.)
In the early 1890s attention focused upon the rays which, at the highest evacuations, stream out from the negative pole (cathode) of the discharge tube and produce a fluorescent glow of its glass walls. Philip Lenard, working in Germany, greatly intensified the physicists' interest in these "cathode rays" by bringing them out of the discharge tube through a hole sealed with metal foil. (See object ID EM.N-08517, Discovery of electron display apparatus with cathode ray discharge tube.)
Between 1897 and 1899 J. J. Thomson, Director of the Cavendish Laboratory, Cambridge University, produced convincing evidence that cathode rays were universal sub-atomic constituents of matter, of identical mass and electric charge. This was the discovery of the electron.
J. J. Thomson devised many discharge tubes in his study of cathode rays. The one with the greatest import, both for his research and for experimental techniques in the emergent field of atomic physics, effected the deflection of the cathode rays by electric and magnetic fields simultaneously. By this means Thomson measured the ratio of the charge to the mass of the "rays,” and established that they were particles, much lighter, and presumably much smaller, than atoms.
